Train working systems – II
Block & Non-block working

Block System

Q. What is the Absolute Block System of train working?

'Absolute Block' refers to a system where the the track is considered to consist of a series of sections, such that when one train is occupying a section of track (the block section), no other train is allowed to enter that section. In addition, no train can enter an empty block section without first securing the permission of the station in advance. This is the most widely used system for ordinary train routes.

A station or signalbox controls a block section in one direction (from its rear), and no train may enter that block in that direction without permission from that signalbox (the station or signalbox is said to accept or receive the train). When a train has been accepted, no other trains can be accepted on that block section until it has left that block section.

Obviously the two signalboxes at either end of the block section have to tightly coordinate their actions, especially in the case of block sections that allow bidirectional movement on a single line. The permission to enter the block may be in the form of a physical token carried by the train crew while the train is in the block, or may be implicit in the aspects of signals governing access to the block.

The block section is usually taken to be the section of track from the most advanced signal controlled by the station in the rear (usually the starter or advanced starter signal) , and the rearmost signal controlled by the station ahead (usually the home or outer home signal).

Sometimes a long stretch between two stations may be formed into two or more block sections (intermediate block) to increase track utilization. The same principle applies in receiving a train from one intermediate block section into the next one. The signal controlling entry to an intermediate block section may be operated by staff at one of the stations, or may have a small signalbox (block hut) where the signal is located.

In automatic block territory, however, there may be several block sections between stations; the signals protecting entry to these sections are controlled entirely by the movements of trains on the sections, as detected by track circuits.

The portions of track that lie between the rearmost outer signal controlled by a signalbox of a station to the most advanced starter signal controlled by a signalbox of the same station are said to be within station limits, or to form the station section. Thus, as one goes along the track, one passes from the station limits of one station, through one or more block sections, and then enters the station limits of the next station. The act of admitting a train into station limits is termed reception, and the act of sending it out of the station is termed despatch.

The restriction on more than one train or rake occupying a block section is stringent and can be lifted only in very special circumstances, some shunting operations, for repair and maintenance work, or emergency operations. For instance, a material train may be sent into a block section that already has other departmental vehicles on it. A traffic train may be sent into a section that has an inspection trolley on the tracks. In all such cases, appropriate caution orders are issued to the drivers of the trains involved, and the driver of any vehicle that is proceeding into the block section in contravention of standard block working rules must carry the appropriate authority to proceed.

Within station limits, however, trains may be moved around by the signalman or station master without reference to other stations or signalboxes, and in fact, depending on the rules for station working in effect for the station, shunting operations, calling-on signals, etc. may allow more than one train to occupy a section of track within station limits. There are many stations where two full trains are routinely moved to the same track, to use the same platform (see the trivia section).

Station working rules are determined for each station in consideration of its peculiar track layouts and facilities. Movements within station limits are generally restricted to a lower speed (15km/h) except for through trains on the main running line(s). Analogous to station limits, yard limits are the regions associated with marshalling yards, sheds, etc. where the requirements of shunting or other activity make it desirable to relax the requirements on the number of trains or vehicles occupying a section of track.

Block working, or the method used to implement the block system, can be of several kinds, noted below.

Block instruments (see below for details) control the coordinated movement of trains on the block section; the block instruments of the two stations or signalboxes at either end of the block section are electrically interconnected for this purpose. (Normally a simple audio-frequency modulated code transmitted over trackside or underground wires is used for this purpose; earlier block instruments were connected directly (by DC voltages).) In physical token block areas, the block instrument is also the device that dispenses the tokens, and hence is also known as the token instrument.

Block instruments are quite old. However, in some areas with light traffic, block instruments were not used. Instead, magneto-driven telephone communication was provided between adjacent stations to allow coordination of signals (also known as telephone block.

In areas with track circuits (previous page) block working may be accomplished without block instruments by using the information from the track circuit to coordinate the aspects of signals using electric or electronic circuitry.

In automatic signalling areas, block working is handled by the track circuits connected to the signalling system such that the movement of the trains controls the signals.

Block working can be done with axle counters too. Here, the task of deciding whether a train has entered a section without parting can be automated by counting the axles as they pass the last signal from the dispatching station and comparing with the count of the axles at the home signal of the receiving station. If they match ('last vehicle proving') the previous block can be opened for the next train. More often, though, axle counters are used in conjunction with track circuits simply to detect the presence of a train on a section for block working.

Communication between the two signalboxes these days is often by telephone, but in the past telegraph instruments that sounded bell signals were universal, and these are still used a lot today. The bell code used by IR is described below.

IR's classifies stations according to their role in the block system and their rules for issuing Line Clear indications. See the section on stations for details.

Also see the extracts of IR General Rules on train working.

Q. What's a token system or a tokenless system?

A token block system uses a physical item (the token) of some sort, such as a ball as in the Neale's Ball token system (see below), which is physically carried or manipulated in some way to indicate permission for a train to enter a block of track. A tokenless system, such as may be obtained by a combination of interlocking and semaphore signals, does not require the train crew to carry or manipulate anything tangible to enter or leave a block of track; they follow the signals and interlocking handles the rest.

(There are also token systems where the 'token' is an electronic radio signal with a code on it that is transmitted to the loco; such systems are not in use in India. E.g., Radio Electronic Token Block working in the UK.)

Q. What kinds of block instruments are or were used in India?

Neale's Ball Token Instrument
Neale's Ball Token Instrument
(Click for a larger view.)

In India, the most prevalent form of physical token system to be found in recent decades is the Neale's Ball Token System and variants such as Neale's Voucher Block, Neale's Tablet Token, etc. Other systems such as Theobald Key Token, Webb and Thompson electric token staff, Tyer & Co.'s key token instrument (one-wire three-position instrument and others), etc., were also widely used.

Robert's Key Staff instruments were used to control single lines on the Mysore State Rly. and the Madras and Southern Mahratta Rly. Neale's Ball Token came into wide use in the interbellum years just before Independence. Mr Neale, by the way, was an engineer on the GIPR. Most early block instruments followed British designs and practices.

Neale's token system was innovative when it was introduced, as it combined, for the first time, the token handling system with the block instrument operation into a single system.

Tokenless block instruments aim to do away with the time-consuming process of manually exchanging tokens. Tokenless operation on single lines is more recent, having started in the 1970s. Among tokenless systems used today by IR, common types of block instruments include push-button tokenless single-line block instruments made by the Signal & Telecom Workshops of SR at Podanur, Diado single-line tokenless instruments, Kyosan single-line tokenless instruments, etc.

Tokenless working of double line sections is much older, because of the inherent safety factor of not having to worry about opposing train movements on the same line. For double-line sections, tokenless instruments made by a collaboration between Siemens (Germany) and General Electric Co. (UK) were in wide use; these are still known as 'SGE' instruments although they are now manufactured at the Podanur workshops.

There are also the so-called Double Line Lock and Block instruments (see below for Lock and Block working) which rely on a mechanical route-blocking arrangement activated by the presence of a train, (the wheels depressing a treadle that activates the interlocking to ensure only one train is on a block section at a time — or indeed, even achieving interlocking between signals under the same cabin). Early on different kinds of Lock & Block instruments were in use on heavy-traffic lines in India; instruments were from Sykes & Co. and other manufacturers.

Q. How do the block or token instruments work?

In manual block working of double lines, the block instruments that connect the two signalboxes (and which dispense the tokens in the case of token systems) have to be coordinated by the signal operators at the two signalboxes.

The signalbox that accepts a train into its block section must first set the Line Clear indication on the block instrument which is reflected in an indication in the block instrument of the signalbox in the rear. The signalbox in the rear cannot pull off signals allowing any trains to proceed into the next block section until such a Line Clear indication is received. When a train enters the block section, the signalbox in front sets the block instrument to Train on line. When the train passes the signalbox in front, the block instrument indication is set to Line Closed (also known as Line Normal or Line Blocked). When there are no trains expected to be approaching or occupying the block section, the normal state of affairs is for the indications on both block instruments to be Line Closed.

In single-line token block working, where a train may be accepted in either direction on the same track, instead of Train on Line there are two indications Train Coming From and Train Going To, which are set by the receiving and sending stations appropriately. In sections with track circuits, the cooperation of the receiving station is not required to set Train Going To, whereas in manual sections the receiving station must set Train Coming From first before Train Going To can be set by the sending station.

Tokenless Block Instruments With tokenless block instruments, the starting signal is the sole authority to the driver for the train to enter the block section. Therefore, before the last stop signal of the station can be pulled off, the previous train (in either direction for a single line) must be verified to have cleared the section and the signals must have been set back to danger.

Single-line working has the obvious complication over double-line working, in that the direction of working needs to be established for each train. In particular, no permission should have been granted for a train in the opposite direction. A common mechanism used is to electrically interlock the block instruments on either side, so that the block instruments from where Line Clear is given is automatically changed to Train Coming From. Only then, and with the cooperation of both station masters, can the other block instrument be changed to Train Going To. This may also be interlocked with a Hepper's Key Instrument or an Interlocked Key Box to release a key for the last stop signal of the station from where the train is departing. Both of these can be ensured (as with Lock and Block working, see below) by providing continuous track circuiting, or by providing entry and exit detection using detectors. Alternatively, signals can be interlocked such that the last stop signal can be pulled off for one train at a time by block acceptance or direction lever control.

Push-button type tokenless block instruments operate in Open Block mode (see below) where either station master can obtain Line Clear after the previous train has arrived intact. With handle-type and other block instruments, both station masters' cooperation is needed before Line Clear can be obtained.

In tokenless single-line sections, the Train on Line indication is retained in addition to the other two. The block instruments may be automated in sections with track circuiting, so that Train on Line is set automatically when a train enters the block section.

The block instrument usually also dispenses one or more shunting keys which allow interlocked operation of shunting signals between the first and last stop signals of the station, and may similarly also dispense other keys or tokens that allow operation of particular points, connections, and signals so as to ensure that a line that has to be isolated really is free of potential obstruction from other lines.

More recently, these different kinds of token instruments have been coupled with electronic devices known as the Universal Fail-safe Block Interface (UFSBI) which accepts input from these block instruments and reproduces the status of the instrument at the other end over a separate channel entirely (usually on the voice channel or a spare channel of a quad cable, OFC, or radio link), to provide additional safety interlocking and fail-safe capability of the token instrument operation. UFSBI devices can work with Neale's token instruments, handle type token instruments, tokenless single-line Diado instruments, and tokenless double-line SGE instruments.

Q. How is the Neale's ball token system or other physical token system worked?

This system is primarily used for single line routes (although it is seen on others too, e.g. MS-KMU-TPJ). The ball token system relies on the crew's physical acquisition and carriage of a small metal ball to mark a train's permission to enter a section of track. The ball is dispensed by a Token Instrument at either end of the block. The two instruments are interconnected so that at any given time, one and only one ball can be handed out from either of them, ensuring that only one train from either direction can enter the section of track ('block') controlled by them.

The token is dispensed by the token instrument only when the block indication is set to Train Going To (see above) after Line Clear has been granted and Train Coming From has been set by the receiving station. While the ball token is carried by a particular train's crew, no other train may enter that section of the track. The token instruments at either end of the block will not give out any tokens until the token carried on the train currently in the block is returned to one of the token instruments. The block indication at the sending station cannot be changed from Train Going To until then. This is usually supplemented by mechanical interlock systems for the points on successive blocks of the tracks; changing the points at one block causes a notification at the signal box for the next block.

The ball token may be engraved with the station codes for the stations at either end, and usually also has a serial number. IR rules require the loco driver or assistant driver to enter the serial number in a register carried in the locomotive cab. In some variants of the ball token, such as tablet token systems, the token is so designed with different patterns of grooves on it that it cannot be inserted into the token instrument of the next one or two block sections beyond the section for which it was intended (in case it is carried beyond the receiving station in error.

A bell or other telegraph mechanism between signal boxes provides for communication of a few basic messages, such as whether the train is on a block, whether it has left the block, whether permission is granted to enter the block, whether a train has had to stop for an emergency in a block, whether it has had to reverse direction, etc. See the bell code below for the full details.

When a train stops at a station, the driver or assistant driver relinquishes any token they might have carried for the previous section, and picks up the token for the next section.

In case the train is 'passing through', i.e., not stopping at the station, the assistant driver puts on a thick arm protector and leans out of the loco. One of the station staff carries a reed and leather hoop about two feet in diameter with a pouch which holds the token. The assistant driver in one rapid move drops the older token and picks up the fresh token from this man standing on the ground. Then the assistant driver shows the green flag to the station master as well as honks the horn to indicate a good 'pick up'. Then he shouts the token number along with the section name to the driver who enters it in his log. The token is picked up in the night in the same way, except the man from the station also carries a flaming kerosene soaked rag or lights a naphtha flare, to indicate his position in the darkness. A short (3' - 4') upright piece or rail or post, usually all white, sometimes with a stripe or two, and located close to the tracks, marks the position where the person handing out the token has to stand for the token exchange.

Picking up a token by hand is generally done only at low speeds. At higher speeds the hoop holding the token is actually mounted on a pick-up stand; the person on the ground stands next to it with a green flag. A pick-up rod on the side of the loco is positioned in just the right manner to snag the hoop and lift it off while the train is in motion. Impromptu "shock absorbers" made of used hose-pipe lengths were used on the side of the loco to prevent the token from being bumped around excessively thereby possibly breaking the hoop or pouch.

Q. What happens if lots of trains go in one direction in a section that uses physical tokens, and one station then runs out of tokens?

As may be deduced from the description of token block systems above, there is a problem if the traffic is not symmetric in both directions between the two stations at the ends of a block section. Eventually, one of the stations will run out of tokens to dispense to trains. This condition is known, not surprisingly, as a token exhausted condition, and results in all traffic in that direction being suspended until tokens (recovered from the other station) can be refilled in the empty block instrument.

In order to avoid this situation, every so often (depending on the traffic), tokens are taken from the station that is receiving an excess of them and returned to the other one (which is dispensing more than it receives); this is known as token balancing.

Q. What is the bell code used to communicate between signal cabins for block working?

IR's bell code has some points of correspondence with systems used in the UK in the early part of the 20th century. The system of bell codes to convey messages from one signalbox to another is given below. An 'X' indicates a short strike on the bell, and a dash indicates a pause.

Bell Codes
Message Code
Attention X
Line Clear Inquiry XX
Train entering Block Section XXX
Train leaving Block Section, Obstruction removed XXXX
Cancel previous, Cancel last (given in error) XXXXX
Danger or Obstruction XXXXXX
Stop and Examine Train XXXXXX-X
Train passed without tail lamp or missing tail board XXXXXX-XX
Train Divided XXXXXX-XXX
Runaway Vehicle (single line, or double line wrong direction) XXXXXX-XXXX
Runaway Vehicle (double line, right direction) XXXXXX-XXXXX
Instrument test XXXXXXXXXXXXXXXX
(16 beats)

The acknowledgement for each of these messages is given by repeating the same message immediately.

Q. What are single line block, twin single line block, etc.?

A single line block system has just one track, which may be used for traffic in both directions. Signals are provided for both directions, and the token exchange or other system of interlocking works for traffic going either way. The basic principle is to restrict access to a block of track to a single train at a time, whether going in the up or down direction. [10/02] E.g., Palghat - Podanur section which is one of these increasingly rare single-line sections with bidirectional traffic.

In a double line section, each line is dedicated for a given direction, so that the up and down tracks are separate. However, to improve traffic throughput, sometimes the twin single line system is adopted, where either of the two lines in a doubled section can be used for bidirectional traffic. Again, as with a single-line bidirectional system, signals are provided for both directions on both tracks. Just before or after a station, track branches to switch from one line to the other are provided, so that traffic on either line is not held up if one of the lines is occupied at the station. The Coimbatore - Palghat (perhaps just the Podanur - Palghat part?) section is an example of twin single-line, as are Morappur - Salem, Dansihpet - Lokur, Yeshwantpur - Bangalore City, etc. These sections tend to have a poorer safety record.

A few sections with heavy traffic have multiple line systems. In a triple line system as is found in some ghat sections where it is necessary to have multiple lines to segregate slower and faster traffic but there is no room to build four tracks, three tracks are used, and the middle track then acts as a bidirectional single line. In a quadruple line there is a pair of up tracks and a pair of down tracks. Sometimes these may allow for segregation of fast vs. slow traffic or suburban vs. long-distance traffic, but not always.

Q. What is an Intermediate Block Section?

Intermediate Block Sections are provided to increase track usage in areas with absolute block operations where the distances between successive stations are large, causing each block to be very long. The heart of the absolute block system is the idea of only permitting one train ever to be on a block at any time; however, if the block is very long, this restriction reduces the possible traffic on the route.

An intermediate block section is an additional block section which does not necessarily correspond to any station, and which is provided solely to decrease the block lengths. Such intermediate block sections are generally used if block lengths would otherwise be more than about 10-15 km. IR prefers block lengths to be on the order of 4 to 8 km, under normal circumstances.

Normally an intermediate block section is guarded by a single stop signal (and perhaps a distant) which is similar to the Home signal for a station. Once a train has crossed the intermediate signal another train may be allowed into the preceding block, just as with the block sections for stations. An intermediate block is usually controlled by the signal box at the preceding station. E.g., between Dahanu and Gholvad on WR is an intermediate block section, which is controlled by Dahanu in the Down direction and by Ghovad in the Up direction.

Many intermediate sections have automatic signals using track circuitry to detect the presence of a train; however, manual operations are not uncommon. An 'intermediate block post' is a station (a class 'C' station) at the boundary of an intermediate block section. Normally only a stop signal and a permissive signal are used for entry to the intermediate block.

An intermediate block signal is usually provided with a callbox for the driver of a train to talk to the station master or signalman of the controlling station in case the signal stays on for long, as the location of the signal may be well out of visible or audible range of the station, making it hard for the presence of the train to be noticed by the station.

If the signal is defective, the driver informs the station master of the station in the rear (controlling the intermediate block), who then obtains a Line Clear notification from the station master of the next station (with exchange of private numbers) and then authorizes the driver to proceed past the defective intermediate block signal (again with exchange of private numbers). If the phone itself is defective, the driver informs the guard and can then pass the intermediate block signal after stopping at it for 5 minutes, and proceeding at 15km/h (at 8km/h in low visibility) to the next station. Once the next station is informed of the defective signal, the station in the rear is also informed of the situation and thereafter the entire stretch between the two stations is treated as a single block section. At that point following trains from the station in the rear will be allowed into the block section with written authorization to pass the defective signal.

Intermediate block sections have no significance for crossing or precedence of trains (express/passenger, freight/passenger) — they are purely provided to decrease the headway or spacing between successive trains. Usually there is no interlocking of any sort provided (i.e., there are no points to route a train differently as the intermediate block signals change, there are no sidings or loops at the signals).

Q. What do the terms 'block station', and 'halt station' mean?

A block station is a station (along with its signalbox) associated with one end of a block section. Each block section has a block station at either end; the block stations control access to the block in either direction. A halt station is a station that is not associated with the end of a block section and has no associated signalboxes and no role in controlling the entry of trains into a block section. These usually serve for picking up or dropping off passengers, etc. at intermediate points in the middle of a long block section. The train proceeds on the flag or lamp signal given by the guard of the train; the duration of the halt is prescribed in the working timetables.

Q. What is a 'block hut' or 'block post'?

A block hut or block post is a minor station (usually Class C; see section on stations) on a section with light traffic, which does not have the full complement of home and starter signals to control the reception and departure of trains. It often has just a single stop signal, and a permissive signal (a distant) to its rear. Line Clear is not granted to a train until the whole of the last preceding train is known to have passed (complete -- without parting) at least 400m beyond the home signal and is known to be continuing its journey. An intermediate block post is a Class C station similar to a block hut on a double line, however in this the signal corresponding to the block post is actually controlled by the signal cabin at the station to the rear, and the intermediate block post is not manned at all. See intermediate block section. The terminology 'Block Hut' in contrast to a block post is often taken to imply that the station or halt is manned.

E.g., on the Howrah - New Delhi line, the Dhanbad - Mughalsarai section (especially around Gurpa - Gujhandi) has many block huts, in some cases with practically every other station or halt being a block hut.

Q. What is Lock and Block?

Sykes Lock and Block
Sykes Lock and Block instrument
(Click for a larger view.)

Lock and Block is a refinement of the Absolute Block system which is often used in areas with power signalling. (Although there is no direct connection with power signalling, the availability of electricity makes it possible.) It is especially relevant on double lines where entry of the train to the block section is controlled by taking off the last stop signal of the station in rear. With Lock and Block, the last stop signal cannot be taken off unless line clear has been received from the station in advance, and furthermore, once taken off the signal cannot be cleared a second time with the same line clear authority. In addition, the entry of the train into the block section replaces the last stop signal to danger. Lastly, line clear cannot be given for a second train until the first train has moved an adequate distance ahead of the last stop signal (and the signals have been put back to danger). This does not ensure that the entire train has passed, nor does it guard against train parting just beyond the signal, but does provide some additional safeguards over the basic token-based block operation. Track circuits or axle counters can be used to detect when the train has passed an adequate distance beyond the last stop signal. In early days - starting from the late 19th century, Sykes Lock and Block devices were commonly used to implement lock-and-block operation.

Q. What is Open Block?

The basic token-based Absolute Block system described above has three states: Line Clear, Train on Line, and Line Closed. (Of course, on single-line systems there may be Train Coming From and Train Going To instead of Train on Line). This is also known as Three-Position Block, or Closed Block, because the fundamental assumption is that the block section is closed as the default state, and after a train has been received intact after a Line Clear, the line must go to Line Closed, and a second Line Clear is needed for any further operation. In the Open Block system, the block section reverts to Line Clear after Train on Line, after the train has been received intact. In other words, the default state of the line is open, and either station may send a train on the line without the prior permission of the other station. This, obviously, contradicts a basic principle of Absolute Block working that the station in advance must give permission for a train to enter the block section. The advantage is that some time is saved in not having to ask for and obtain Line Clear - but safety is reduced and this requires careful and vigilant working. Sometimes this is combined with Controlled Manual Block (see below). Push-button type token instruments often work in Open Block mode - either station master can obtain Line Clear directly, after the previous train has been acknowledged to have arrived intact and the instrument has been normalized.

Q. What is Controlled Manual Block?

Controlled Manual Block is a system where a manually operated block instrument is used for obtaining the Line Clear permission from the station in advance; however, the clearance of the section must first be verified through track circuiting. The block instrument is connected to the track circuits, and it is not possible for Line Clear to be granted unless the track circuits verify the entire block section to be clear of obstruction. Axle counters can be used instead of track circuits as well.

Q. What's a Moving Block system?

Moving block is a more advanced system usually associated with ATC (Automatic Train Control). As of [1/00], it is not in use anywhere on IR. In moving block systems, a train's position and speed are communicated to control equipment which then computes the appropriate headway to be maintained between and other trains on the same track, and accordingly manipulates the signal aspects and speed controls for the trains on the track. The fine control over the speeds and positions of the trains allows for tight headways and correspondingly high utilization of the track.

Q. What is Automatic Block Signalling? (What is the Automatic Block system?)

In Automatic Block Signalling (ABS) the signals are automated and operate in conjunction with track circuiting or other means of detecting the presence of a train in a block section. [2/05] As of March 2003, IR had 3,606 kms of track under the ABS system.

When a train enters a block section, the stop signal protecting that block changes automatically to on or the Stop aspect. As the train moves ahead out of that block and into the next block, the signal aspect changes automatically to Caution. In multiple-aspect signalling, when the train is 2 blocks ahead the aspect then changes to Attention, and then to Proceed when the train has passed 3 blocks ahead. In 3-aspect signalling, the aspect changes to Proceed when the train is 2 block sections ahead. In automatic block territory 2-aspect signalling is not used. Each change of a signal to a less restrictive indication requires the train to have moved an adequate distance in advance of the signal.

The gray, white, or silver boxes marked 'LOC CAB' found by the side of the tracks contain the circuitry to accomplish the automatic signal transitions. Any number of automatic block stop signals may be provided in between two block stations; thus with this system, the two stations do not define the ends of a single block section as is usually the case with manual absolute block working (excepting, of course, the case of intermediate block sections). Minimally one automatic stop signal is provided to the rear of a block station's first stop signal.

The Home and Starter signals of a block station must be manual or semi-automatic (see below) even in automatic block territory, and cannot be fully automatic (however, they may still be operated remotely from a central location if the station does not have its own control cabin, as with the Mumbai area sttaions that come under the TMS (Train Management System) centralized traffic control system).

Note: Automatic Block Signalling is an American term and is the same as 'Track Circuit Block' in British terminology; the American influence starting from the 1930s and through the war years on signalling and interlocking developments in India probably led to this usage in India.

Automatic signals are normally always in the clear position (Proceed aspect), except when the next signal ahead is manually operated, in which case the normal aspect shown is either Caution or Attention. Automatic block signals are provided with a small circular plate marked 'A' (black on white) on the post of the signal or next to it. In contrast to these, manual signals are worked by the signal operator and are normally always in the on position and have to be explicitly pulled off by the operator.

There are also semi-automatic signals which can work either as automatic signals or in manual mode. When working in manual mode, a semi-automatic signal assumes the on position automatically when a train occupies a block section ahead of it just like an automatic signal, and can be manually pulled off only after block sections ahead are clear. These are provided with a small circular plate marked 'A' (black on white) which is lit by a white lamp when the signal is working as an automatic block signal and not lit when the signal is being worked manually. The reason for providing such dual-mode signals is that sometimes the signals control diverging lines that are used only occasionally or only at certain times of day (e.g., goods shed lines, industrial sidings). In such cases, it is preferable to keep the signal in automatic mode for the bulk of the heavy traffic passing straight through on the main line, but to change it to manual mode when traffic needs to be diverted off the main line. In addition, semi-automatic signals are also provided at stations with loops where the signals can be set in automatic mode for trains running in quick succession straight through on the main line, and only changed to manual mode for trains that need to be received on the loops.

Gate stop signals in automatic block territory are provided with both a 'G' marker as noted above for gate signals, and also an 'A' marker (white on black). Calling-on signals in automatic territory have the 'C' marker as usual in addition to the 'A' marker, and these are found only at entrances to stations which have their own control cabins to decide the calling-on aspects.

When approaching a fully automatic block stop signal which is on in automatic block territory, the train must come to a standstill to the rear of the signal, but then in some cases, after waiting for some time (normally 2 minutes, sometimes varies from day to night -- 1 minute in daytime and 2 minutes at night), if the signal does not change aspect the train may pass the signal at danger at a low speed (typically restricted to 15km/h), with the driver alert for other vehicles or obstructions on the track. This is also allowed on the Mumbai suburban networks when an automatic block signal has failed. Incidentally, fully automatic signals often do not appear on the control panel at the control towers.

Delayed Signals: Automatic signals as described above are to be distinguished from delayed signals or time-delay signals which are also sometimes confusingly called automatic signals. These are automatically pulled off after a train has halted to the rear for some predetermined amount of time (using a timer controlled by a track circuit). They are mostly found at brake halts on steep ghat sections. These signals also usually have an 'A' board attached, but they have nothing to do with automatic block working as described above.

Other than track circuiting axle counters are used in some sections to detect the presence of a train in a section and its departure from the section. Intermediate block sections often use axle counters instead of track circuits to detect the presence of a train on the block. Currently [1/01] about 2000 axle counters are used for detection of a train's presence in a section in conjunction with track circuiting, and about 150 axle counters are used for block working by means of automated last vehicle proving.

Q. What is Permissive Block (or Absolute Permissive Block) Signalling?

Normally, in Absolute Block Signalling, the single-line section between points where sidings or loops are provided must be treated as a single block in order to prevent two trains from entering it at the same time. However, this also reduces track utilization in the case of trains following one another in the same direction.

In these cases, a variant of absolute block signalling is used, called Absolute Permissive Block, where for opposing movements (trains in opposite directions), the single-line section is treated as a single block, whereas for following movements (same direction), the single-line section is treated as a sequence of smaller blocks, allowing many trains to be on the single line at the same time.

This is done by having lineside signals at several points along the single-line, which function as normal block signals for following movements; but for opposing movements, all signals for an opposing train turn 'on' ('stop') for the section of track in front of a train that is already in the single-line section. Permissive block signalling is more often used for freight movements than for passenger trains. Sections with absolute permissive block include Gorakhpur-Chupra (NER) and Bongaigaon-Changsari (NFR).

Q. What is the 'authority to proceed'?

Authority to proceed is the formal name for the permission or authorization that allows a driver to take a train into a section of track, in either block or non-block systems of working. It is also known as train working authority. In places where token block instruments are in use, block tokens constitute the authority to proceed. Normally in tokenless block working the authority is implicit in the aspects of signals, although it may be a paper form if signals are defective or missing, etc., or if signalling has been disrupted by equipment failure, floods, etc. In non-block working or under special circumstances, authority to proceed is physically given to the driver on a paper form, or conveyed to him by telephone or walkie-talkie.

Any form of authority to proceed which is in physical form (token, paper form, etc.) is also known in IR rulebooks as tangible authority to proceed, and strict rules govern how it is handled and passed on from one staff person to another.

The following are the most common types of tangible train working authorities in the block system. These are almost always issued on paper forms (except, of course, the block tokens). When these are issued in unusual circumstances (e.g., communications or signalling failure, floods, etc.) in sections where tokenless working is the norm, private numbers must be issued by the station masters concerned along with the paper authority. If communications between stations are disrupted the section controllers can issue paper authority with exchange of private numbers.

  • Block token (Neale's ball token, tablet token, electric staff token, or key token)
  • Paper Line Clear ticket
  • Starting Permit: Allows a driver to take his train on to a block section to begin its journey. Usually references a token number or paper line clear ticket.
  • Authority to pass defective signal
  • Authority to pass reception signal at On (to enter station limits)
  • Authority to pass departure signal at On (to leave station limits)
  • Authority to pass signal at On (gate, automatic, semi-automatic, or manual signal)
  • Authority for train reception on unsignalled line
  • Authority for train departure from unsignalled line
  • Authority to proceed in Automatic Block section during prolonged failure of signals
  • Authority to proceed without Line Clear
  • Authority to proceed without Line Clear in Automatic Block territory
  • Conditional Line Clear certificate (granting Line Clear following the reception of a train known to be en route)
  • Caution Order (may enforce restrictions, or may cancel previous notices. A Reminder Caution Order reiterates a previous one.)
  • Nil Caution Order (notice of absence of special restrictions until the next notice station)
  • Guard's authority to work a part load from a section (in case of a train that has parted)
  • Guard's permission to Driver to proceed with complete or partial train from mid-section to the next station
  • Guard's permission to Driver to return to section from next station, when train has parted
  • Authority to enter block section for shunting
  • Shunting order when blocking forward
  • Shunting order when blocking back
  • Line Clear certificate for trolley / motor trolley / motor lorry
  • Line Clear certificate (double line) for trolley / motor trolley / motor lorry
  • Authority for trolley / motor trolley / motor lorry to enter block section without Line Clear certificate.

In non-block systems of train working, the following kinds of authority to proceed are used:

  • Authority to Proceed, Following Trains system
  • Pilot-guard Ticket
  • Train-staff
  • Train-staff Ticket
  • Metal Token for One Train Only system

Q. What is a Private Number?

A Private Number is a number, obtained over the telephone or telegraph from the station master of the station granting Line Clear or requesting points to be set or signals to be pulled off. This number is noted on the paper forms such as the Line Clear Ticket or Conditional Line Clear Certificate and can be verified later at the receiving station (the one granting Line Clear) or requesting the signal or points change. This is an additional safety device.

Private numbers are printed or typed up in advance on booklets which are supposed to remain in the custody of the station master or his immediate staff. In theory, it is not possible for anyone outside the station master's office to predict the next private number that will be issued since they are pseudo-random in nature and do not follow any sequence or pattern. Hence, the verification of the private number provides a good confirmation that the action it refers to was performed correctly and not in an unauthorized manner. Under rare circumstances, two consecutive private numbers may turn out to be the same or nearly so; in this case the second one is cancelled and a new one issued by the station master.

In addition to blocking or clearing trains, private numbers can be used to confirm control messages for rerouting trains, permitting unusual movements such trains on the wrong line, issuing new speed limits directly through the control office, or exchanging any other messages between section controllers and station masters. For instance, notifications of temporary speed restrictions, temporary line blocks or power blocks, etc. Another case is that of closing the gates for road traffic at non-interlocked level crossings (in this last case, private numbers are generally used only when the level crossing is on a block section and not within station limits).

Security buffs will note that while the possession of a valid private number shows that the action performed by someone was authorized, it is not an entirely fool-proof system. In particular, the system does not guard against impersonation (the person to whom the private number is revealed may not be who he says he is, or the person providing the private number may not be the one authorized), nor does it provide non-repudiation (the person who is given the private number can disown having obtained it) or spoofing (there is no way for the person who receives a number to verify that the number provided is a legitimate private number before acting upon it). Hence, mechanical or other interlocking systems are still used in conjunction with private numbers.

Non-block train working

Q. What other forms of train working are used on IR other than the block system?

IR on occasion uses the following systems of train working:

Following trains system

In this system, trains are worked between two stations by dispatching them one after the other (all in the same direction) at specific intervals (generally at least 15 minutes). The trains are run at specific speeds (less than 25km/h). This ensures that there should only be one train in every 5km stretch in the section between the stations. The maximum number of trains simultaneously present in the section is restricted to 4.

Trains are dispatched only after the station masters of both stations have been in communication and have agreed upon the number and timing of the trains. The driver of a train being dispatched in this system must carry written authorization (Authority to Proceed, Following Trains System) specifying the destination, speed, and details of preceding and following trains on the section.

This system was introduced in many areas as an emergency measure in 1941 in order to cater to the urgent wartime needs of the railways in India. It is still used in some areas when there is a requirement for large unidirectional movements of many freight trains (also known as the Corridor System).

Pilot guard system

Here, a pilot guard, a specially authorized railway official, accompanies the train (he is said to pilot the train). As above, a train is not dispatched until at least 15 minutes (usually) have elapsed since the preceding train, and speeds are limited to 25km/h.

The pilot carries written authorization (Pilot Guard Ticket) with the destination, departure time, and speed of the train, which has to be handed over to the station master at the destination station. Only one pilot guard is allowed to be on duty for a section at any given time, of course. Rarely, the driver or guard of the train may carry the ticket and the pilot may not actually accompany the train. (Usually the case if two or more trains are to leave together from one station under this system, in which case the pilot guard travels on the last one.)

The pilot guard system is used when it has been agreed upon earlier that trains will be dispatched in a specific direction using the system, but the precise times are not known in advance and communication cannot be established with the destination station immediately prior to sending the train out.

Train-staff and ticket system

This system is used when it is necessary to send trains in both directions between two stations on a single line. A single train staff is used, and trains may only be dispatched from the station which has physical possession of the train staff.

If several trains in one direction are to be dispatched, they are spaced in a manner similar to the Following Trains system, and each driver except that of the last train is handed a train staff ticket authorizing him to proceed. (Technically, the driver is also supposed to be shown the train staff before proceeding). The driver of the last train physically carries the train staff along with him in the locomotive. (This is similar to the pilot guard system except that the train-staff replaces the pilot guard's physical presence on board the locomotive). No other train can then be dispatched from that station.

The driver then hands the train staff over at the destination station. From that point onwards that station can start dispatching trains in the other direction in the same manner. Hence the train staff acts as a guarantee that trains are not simultaneously dispatched in opposite directions. E.g., Tilwara - Tilwara Mela (NR).

One train only system

In this system, as the name implies, there is only one locomotive being worked at a time in the (single-line) section between two stations, or a single station and a spur section of track with no station at the other end. One of the two stations at either end (or the sole station if there is only one) is designated the base station for the section. The loco may be dispatched light or with a vehicle load in either direction at any time.

The driver carries a metal token given to him by the station master of the base station. The token may also be in the form of a wooden baton. The token identifies the section for which it is valid. Of course, only one such token should ever be in use at any time on the section. (Rarely, written authorization is provided instead of the token.) This system is usually employed only for short single-line spurs, and not on through lines. E.g., Batala - Qadian, Nawan Shahr - Rahon, Ratangarh West - Sardar Shahar, Garhi Harsuru - Farukhnagar.

A variant allowing one locomotive at all to be present (no other locomotives are even allowed to be brought into the station limits) called One Engine Only system was in effect between Rajka Ka Sahaspur and Sambhal Hatim Sarai. (All these are on NR.) This system is also used sometimes on new tracks where pointwork or signalling construction is not yet finished.

In Jan. 2004, the second phase of the Chennai MRTS began operation with the One Train Only system between Thiruvanmiyur and Thirumayilai because points for turnouts / sidings were not yet ready across a stretch of 9 stations.

Section Clear

In this system, permission to approach a station is given to a train only when the line is known (manually verified) to be clear up to the first stop signal of the station. The driver is given written authorization as the authority to proceed.


Also see the extracts of IR General Rules on train working covering these non-block systems.


See the section on operations for details of caution orders, line blocks, and block protection in unusual situations.

For details of semaphore and colour-light signals, etc. see the section on signalling systems used by IR and other sections referenced from there.

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