Signal Aspects and Indications — Principal Running SignalsOn this page
General information on signalling systems is found in another section. This section focuses on the main running signals and the aspects they display.
Note: Information on semaphore signalling is mostly historical and is provided for the sake of completeness. Almost all of IR now uses colour-light signals.
A stop signal governs access to a block section and ordinarily may not be passed when it is at its most restrictive indication (the on position, which shows the stop or danger indication for these signals). That is to say, when on, its interpretation is ‘stop dead’.
Under some special circumstances, a stop signal may be passed at slow speed after the train has been brought to a standstill at the rear of the signal. This is commonly allowed in automatic block territory where the driver can proceed after waiting for a minute or two. In most other cases, the driver must obtain permission to proceed over a telephone callbox at the signal, or must have written authorization to ignore or pass the signal.
Automatic stop signals and delayed stop signals (see the section on block working) are provided with a circular plate marked ‘A’ (black on white).
Colour-light: The stop signal may have two (green above red), three (green-yellow-red), or four lamps (yellow-green-yellow-red). Aspects are as shown below.
Semaphore: The semaphore arm of a stop signal is red in front with a white stripe near the end, and white in the back with a black stripe near the end. The arm is square-ended. Signal aspects are as shown below.
Stop signal indication summary:
- 2LQ, MLQ, 2CL: Stop, Proceed
- MAUQ, 3CL: Stop, Caution, Proceed
- MACL: Stop, Caution, Attention, Proceed
Usually the signals are set up at sufficient distances so that, for instance, a train arriving at a Caution signal at the maximum speed for the section can safely brake to a halt before the next signal which is at Stop. Older locos especially hauling vacuum-braked rakes or long freight rakes should be able to slow down sufficiently when they reach a signal at Attention so as to be able to halt after reaching the next signal at Caution; but the newer locos hauling air-braked rakes can reach a signal at Attention at the maximum speed for the section and proceed through it without slowing down and still brake safely if the next signal is at Caution.
If the distance between the signal at Caution and the signal at danger is less than the safe braking distance, the signal to the rear displaying Attention also serves to alert the driver that the train may have to be slowed to restricted speed when it reaches the next signal.
A signal that is to the rear of a signal protecting a divergence cannot show an indication less restrictive than Caution or Attention when the points are set at the divergence for any line other than the main line. (The divergence should normally also be indicated by the use of a route indicator.) This Caution or Attention indication may be repeated further in the rear if the distance to the divergence is insufficient to permit a train to slow down to the appropriate speed for the divergence. The Caution indication is also used to indicate track sections with speed restrictions. The Attention indication may correspondingly be displayed by the signal to the rear of the signal guarding the approach to a curve or a divergence or section with speed restrictions.
The starter signal (see below) may show Attention or Caution to provide permission for a train to leave a station, instead of the Proceed indication.
Stop signals are of the following types.
This is the first stop signal on approach to a station without an outer home signal. It is not optional. The signal guards entry to the station limits ahead from the block section in the rear and appears before all connections to the line (branches, loops, etc.) at the station.
A home signal at Caution indicates that the train may have to stop on the line before leaving the block, or that the train has to slow down to a particular speed in order for the starter signal at the entrance to the next block to shift to Proceed. A home signal is also set at Caution for temporary or permanent speed restrictions within station limits. An optional (electric) numeric display on the post of this signal is usually an indication of the platform to which the train will be routed.
For stations with multiple lines where a train may be received (i.e., main running line and loop lines), normally home signals are provided with route indicators (‘feathers’) just before the diverging points to the various lines, to indicate for which line the points have been set for the train to be received. In semaphore signalling as many number of sets as the number of receiving lines is to be provided.
Also in semaphore signalling, the main line home signal is placed above any others; the lower signals refer to lines diverging to the left or right of the main line according to their position with respect to the main signal. Such signal arrangements are also referred to as bracketed home signals. Bracketed homes require interlocking between points and signals.
Outer (Outer Home)
To increase track utilization, or to provide better control over approach to station limits, additional signals may be provided to the rear of the Home signal. An Outer Home signal (also known simply as an Outer), to the rear of the home signal, is very common. The outer may be at Caution to indicate speed restrictions further ahead, or if the home signal is at Stop.
Intermediate home signals may be provided between the outer and home in some cases to provide finer control over train movements on approach to station limits.
This stop signal is provided on intermediate block sections which are block sections created by subdividing a long block section between stations; there isn't necessarily a separate station or route junction at the point. (If there is a station, it is an intermediate block post or halt station.) An intermediate block signal simply protects the block section ahead of it in a manner similar to a starter signal. A circular marker with ‘IB’ (black on white) is fixed to the post below the signal. The signal is controlled by the cabin of the station to the rear if the intermediate block post is not manned.
This indicates which of two or more diverging routes have been set, especially in cases where the corresponding Home or Outer or other stop signals before the facing points do not provide such indication.
This governs exit from the most advanced section within station limits, and entrance to the block section ahead. It marks the limit up to which a normal train can stand at a station. (Shunting movements can go beyond the starter when intermediate or advanced starters are provided.) Normally it is the last stop signal on departing from a station unless an advanced starter is present. If an advanced starter is provided, the starter may protect facing points to another running line at the same station. Starter signals are provided at most stations, but there are some without them. If the starter is not provided, station working rules prescribe when trains may proceed to the next block section; usually tangible authority to proceed such as a Neale's ball token or paper line clear ticket are needed.
If there are several converging lines exiting a station, each is usually provided with a starter so as to protect each line from fouling the adjacent lines. If a single starter is provided for several converging lines exiting a station (this is rare), it is placed beyond the trailing points of the convergences. In some areas, a starter signal may be set up so that it does not shift to the Proceed indication unless the train slows down to a particular speed (or stops) before reaching it (in such cases the home signal at the entrance to the block is usually at Caution). Shunting cannot take place without special instructions beyond the starter if it is the last stop signal at the station.
Normally the starter signal shows a Proceed indication (green signal) to indicate that a train may leave the station, but in some cases an Attention or Caution indication (double yellow/yellow) may be used to allow the train to leave the station (and make the platform available for another train) but at a reduced speed. On Konkan Railway lines it has been observed (04/2001) that the Attention indication (double yellow) is routinely used for the starter signal.
A starter signal may have additional lamps or signs such as ‘M’, ‘B’, etc. to indicate which tracks the train will depart on (mainline, branch line, etc.), in the case of diverging lines beyond the starter. Multiple semaphores or colour-light signals may also be used (bracketed starters), or route indicators.
This is an optional signal. It is a stop signal provided ahead of the starter signal, and therefore if present it is the last stop signal on departing station limits. The advanced starter allows shunting operations beyond the starter. Normally shunting may not take place beyond the advanced starter. Otherwise the advanced starter, if present, functions just like the starter signal to control exit from station limits and entrance to the block section ahead. It is placed ahead of all trailing points for converging lines exiting the station, and therefore, there is usually just one advanced starter for all the lines at the station.
Intermediate starters may be provided between the starter and advanced starter to split up the section into smaller sub-sections and provide finer control over train movements and shunting operations. Intermediate starters are placed to the rear of the fouling points of the points they protect.
A gate stop signal guards interlocked (or sometimes non-interlocked) gates at level crossings. A circular plate marked ‘G’ (black on yellow) is fixed on the post below the signal. A gate signal may be passed after the train comes to a standstill to the rear of the signal and after waiting for a minute or two. The train may then proceed slowly up to the level crossing, and must then wait for the gateman to direct the train across the level crossing with hand signals.
A gate signal may be placed on the same post as an outer signal, or the two may be combined. If an outer signal is ahead of the gate signal and there is insufficient visibility of the outer signal, the gate signal and the outer signal can be slotted to work together so that the gate signal is never pulled off when the outer is on. In such situations, the distant signal pertaining to the outer home acts as the gate distant.
Note: In very rare instances, if the distance between stations is really short, and the station to the rear needs an advanced starter which would appear in about the same place that the outer home for the station ahead, the two may be combined into one stop signal controlled from both stations. Thus the train effectively moves directly from the station limits of one station into the station limits of the next.
Note: For class ‘C’ stations, the Home signal is both the first stop signal and the last stop signal, as starter signals are usually not provided.
A warner signal is used only in two-aspect signalling (2LQ, MLQ, 2CL). Its purpose is to warn of an approach to a stop signal further ahead, or to advise a driver of the condition of the block section being entered. As such, it is a permissive signal and may be passed when it is in its most restrictive (on) indication, although when it is on the train must reduce speed.
A warner is always set to the on position for a train which is scheduled to stop ahead at the station. A warner may also be provided in 2-aspect territory on the approach to a gate stop signal. Normally warners are pulled off only when the stop signal they refer to is pulled for the main line (highest permissible speed), and not if a stop signal for a divergence is pulled off. There are some other considerations, see below.
The warner is often paired with a stop signal (for example, an outer-warner combination is very common), in which case the warner's indication is never less restrictive than that of the stop signal, and if the stop signal is on, the combination cannot be passed. When the stop signal and the warner are both clear (in the case of outer home signals this is known as 'home double' or 'double home'), the signal may be passed at the maximum speed for that section.
In a combination warner, the stop signal may show Proceed and the warner may be on, to indicate that the next stop signal ahead (usually the home signal, in the case of an outer+warner combination) is on (at Stop).
In some cases, the warner may not be pulled off (see distance considerations below) at all. Allowed combinations are outer+warner, starter+warner (if no advanced starter), or last stop signal + warner (i.e., advanced starter + warner). The mechanical interconnection between the stop signal and the warner in semaphore signalling, which prevents the warner from being less restrictive than the stop signal is known as slotting.
If the warner is by itself, a fixed green lamp is usually placed above it on the same mast (so that technically it is equivalent to a warner below a stop signal which is always clear).
A warner signal may be set up to be permanently in the on position (caution indication). In this case the warner merely advises the driver of a train of the approach to a stop signal ahead or possibly a permanent restriction or problem with the track ahead.
Illustrations covering aspects in both semaphore and color-light systems are shown below.
Colour-light: The warner consists of a two-lamp signal (green above red) fixed below the (2-aspect) stop signal on the same post. A warner without a stop signal has a single green lamp (always lit) above it on the same post, and a small circular plate marked ‘P’ (black on white) below it.
Semaphore: The semaphore arm of a warner signal has a vee-notch (called a ‘fish tail’ in IR manuals) at the end; it is red in front with a white stripe (V-shaped) at the end, and white at the back with a black stripe (V-shaped) at the end. Aspects are as shown below.
Warner signal indication summary:
- 2LQ, MLQ, 2CL (Lone Warner): Caution, Proceed
- 2LQ, MLQ, 2CL (Warner in combination with stop signal): Stop, Caution, Proceed
- MAUQ, 3CL, MACL: Warners not used
A distant signal (also known as a pre-warner) indicates approach to a more restrictive signal further ahead. In IR terminology, the distant is said to ‘pre-warn’ the driver of the indication of the next signal ahead. Examples: The distant signal shows Caution, and the next stop signal ahead is at Stop. Or, the distant signal shows Attention, and the next stop signal is at Caution.
A distant signal may be at Attention if the following signals guard a divergence and the points there are set for a route other than the main line. A distant signal to the rear of signals at a divergence will be at Proceed if the points are set for the main line at the divergence. In that case, the stop signal for the main line may be at Caution. Of course, both the distant and the next stop signal may be at Proceed.
A distant signal is a permissive signal and may always be passed even in its most restrictive indication. A distant signal is analogous to a distant signal that occurs by itself in UK practice. A distant signal is typically at a distance of 1km or so from the stop signal it protects, but this may vary depending on the particular track requirements.
Outer and Inner Distants
In some sections two distant signals may be provided to the rear of a stop signal. In that case, the one further to the rear of the stop signal is known as the outer distant or the second distant, or simply as just the distant signal and the one just before the stop signal is known as the inner distant signal. In such a case, the outer distant can only show two indications, Attention and Proceed, while the inner distant can show Caution as well. Two distants are standard on routes with speeds above 100km/h and where goods trains run which require braking distances over 1km.
A distant signal is usually placed far enough (2km or so) to the rear of the stop signal it protects that when it is at Caution a train at the maximum speed for the section can brake safely to a halt before the stop signal. Otherwise, the Caution indication may be replicated further back by using more than one distant until the rearmost distant at Caution is at sufficient distance from the stop signal.
Distant signals may also be provided to the rear of gate signals, in which case they are known as gate distant signals and have the ‘G’ marker just like gate stop signals. However, a distant signal may act as a distant signal for both a normal stop signal as well as a gate signal.
In rare cases distant signals may be mounted on the same mast as the last stop signal of a station or a gate stop signal. In such cases the distant signal operates with the additional restriction that its indication can never be less restrictive than that of the stop signal.
A distant signal showing the Proceed indication (clear) is also known as a ‘distant green’ from its colour-light indication.
Illustrations covering aspects in both color-light and semaphore systems are shown below.
Colour-light: Distant signals have a small circular plate marked ‘P’ (black on white) mounted on the same post, below the signal (this marker is omitted if the distant signal is mounted on the same post as the last stop signal for a station). The signal itself has 3 lamps, of which the top and bottom are yellow. Aspects: For Caution only the bottom lamp is lit; for Attention both yellow lamps are lit, and for Proceed just the green lamp is lit. The aspects are shown below.
Semaphore: A distant signal has a vee-notch at the end; it is yellow in front with a black stripe (V-shaped) at the end, and white at the back with a black stripe (V-shaped) at the end. At Caution and Attention the semaphore spectacle displays a yellow lamp at night; for the Proceed indication a green lamp is displayed. In upper quadrant territory, an additional yellow light is placed below the signal, on the same post, and is lit when the distant is in the Attention indication, so that at night two yellow lamps are seen.
Distant signal indication summary:
- 2LQ, 2CL, 3CL: Distants not used
- MLQ: Caution, Proceed
- MAUQ, MACL (sole distant or inner distant): Caution, Attention, Proceed
- MAUQ, MACL (outer distant): Attention, Proceed
Difference between Warner Signals and Distant Signals
Although distant signals and warner signals appear to serve similar purposes, there are some important differences between them. Distant signals are generally placed the full braking distance before the first stop signal of a station, whereas a warner can be placed on the stop signal itself (as at a ‘B’ class station). The distant signal indicates the aspect of the stop signal ahead. With a warner, however, the indication is definite only when it is off (‘Proceed’); when it is indicating the caution aspect, it could mean that the home signal is at danger, or that the train may be received on a loop line, or that there is a speed restriction ahead, etc. This means that the driver of the train cannot control the speed of the train as carefully as he can with multiple-aspect signals.
Provision and Placement of Signals
Adequate distance is a term that is used in the context of placement of signals. It generally refers to the safe distance to allow in the placement of a signal to allow for errors and overshooting signals or mechanical failures. For some signals, the adequate distance is the braking distance, also known as the warning distance — the distance a train running at the maximum permissible speed would need to be able to brake to a complete stop. For other signal contexts, the adequate distance is the distance required for the driver to safely brake to the lower speed required ahead.
Overlap is a term used for the adequate distance beyond a stop signal, which is required to be clear of obstructions, before a train can be received at that signal when it is at danger. The provision of overlap reduces the likelihood of collisions if a train overshoots the signal at danger.
Block overlap is the overlap associated with a reception stop signal of a station (home or outer), and is the distance to be provided from that signal to the first facing points of the station (for a home signal without an outer signal), or from the outer signal to the shunting limits of the station or to the advanced starter in the opposite direction. It is usually also the distance to be provided between the home signal and the starter signal, regardless of whether an outer signal is present. If an interlocked level-crossing gate is present, then the outer signal is usually placed at least this distance to the rear of the gate; if separate gate signals are provided, they must be at least the block overlap distance from the gate. Block overlap is usually prescribed to be 400m for lower quadrant or 2-aspect colour-light signalling (originally 1/4 mile in British operation), and 180m for MAUQ and MACL signalling. This can in some instances be lower by special permission from the CRS.
Note that in the case of outer and home signals, the distance between them is often higher than the standard block overlap, by 180m - this allows additional shunting activities to happen up to 180m to the rear of the home signal.
This refers to the overlap to be provided beyond any other stop signal other than the outermost stop signal for the station; the term is especially used for the overlap provided in advance of the starter signal. The signal overlap is normally 180m for lower quadrant or 2-color colour-light signalling, and 120m for MAUQ or MACL signalling. The signal overlap is smaller than the block overlap as it is presumed that a train is generally better under control within the station territory - and there is also a lower likelihood of errors because both signals that the train is moving under (the signal that it just passed that allowed it to proceed, and the signal it is approaching) are controlled by the same authority. (As opposed to the block overlap where the train enters the block section and approaches the outermost stop signal of the station, having received a proceed signal from the previous station.) For the same reason, the requirement that the signal overlap distance be clear of obstructions is relaxed when the train has first come to a dead stop at the signal to the rear.
Thus, the home signal can be taken off only if the signal overlap distance beyond the starter signal is free of obstructions. On a single line, the distance is actually measured from the trailing points, whereas it is measured from the starter signal in the case of double lines. Note that this applies only to trains in motion that are approaching the home signal; for trains at a stand-still at the home signal, the home may be taken off if the line is clear to the starter (double line) or to the trailing points (single line).
Advanced starters are usually placed 180m beyond trailing points.
The warning distance is the distance required to brake a train to a complete stop and is usually the distance provided between a warner signal and the stop signal ahead that it is associated with; this is important in LQ signalling because the driver has to be prepared to bring the train to a halt after seeing the warner at caution.
If a warner is to the rear of a gate stop signal, it is usually never pulled off unless the first stop signal of the next station is at least 1200m ahead of the gate stop signal, regardless of the indication of the gate stop signal. If a warner is provided in a station whose last stop signal is less than 1200m to the rear of the first stop signal of the next station, the warner is pulled off only when the first stop signal of the next station is pulled off.
As above for warners, but the distance in question is 1km instead of 1200m.
3- or 4-aspect signalling: All running signals must be visible for at least 200m. If this is not possible speed restrictions are imposed to the rear of the signal for which visibility is impaired, and repeating signals may also be provided.
Two-aspect signalling: Outer signals have to be visible for 1200m if train speeds exceed 100km/h; 800m otherwise. If a warner signal is provided to the rear of the outer signal, the visibility can be 400m. Lone warners, home signals, and main starter signals must have a visibility of 400m. All other running signals have to be visible for at least 200m. When this cannot be complied with, repeating signals are provided.
Q. What signals are provided at different kinds of stations?
Generally, fixed signals have to be provided at all block stations (i.e., classes A, B, and C), except those operating trains under the One Train Only system. The minimal signal provisions for block stations with manual absolute block working are described here. Additional signals may be always be provided based on local requirements. Note that the requirements below are for each direction of approach to the station.
In 2-aspect territory, a Warner, a Home, and a Starter signal are provided. In other systems a Distant, a Home, and a Starter are provided. On double-line sections an Advanced Starter is also provided. As the Home signal is the outermost stop signal, the line has to be clear for the appropriate adequate distance (block overlap - 400m for LQ, 180m for MAUQ/MACL/MLQ) beyond the home signal before a train is given permission to approach (i.e., before Line Clear can be granted). The Starter is at an adequate distance beyond the Home. The Warner or Distant follow standard placement guidelines (see below). The Home signal may be bracketed.
This arrangement is suitable in cases where traffic passes through rapidly, and advance knowledge of the condition of the block section is required for the driver. With higher running speeds, it is important that the line be clear for a larger distance (including the section of the line within station limits) before Line Clear is given. The first stop signal is necessarily closer to the station (no Outer signal) and this can create constraints - e.g., if there is an approach gradient near the station, making it inconvenient or unsafe for trains to stop at the Home signal. The disadvantage of the arrangement stems principally from the fact that the line within the station between the home and the starter has to be cleared before Line Clear can be given, which limits working flexibility, shunting, and overall traffic flow.
In 2-aspect territory, an Outer and a Home signal for single-line sections, and an Outer, a Home, and a Starter for double-line sections. Warners are provided if train speeds exceed 50km/h. In other systems, a Distant, a Home, and a Starter signal are provided. The main line Home signal usually has a Warner on the same post in modified lower-quadrant working. A shunting limit board is provided in some cases, or an Advanced Starter instead of it. As the Outer signal is the outermost stop signal, the line has to be clear for an adequate distance beyond it (400m for 2LQ, 180m for MLQ, MACL, MAUQ) for Line Clear to be given. A warner is provided in case the run-through speed for the station is over 50km/h.
At single line stations, this arrangement does not provide flexibility for shunting compared to an ‘A’ station, primarily because the shunting activities are still restricted to the portion of the line in advance of the home signal if Line Clear has been given. Therefore, to allow flexibility in shunting activities, the Outer signal is usually placed an additional 180m (beyond the block overlap distance) to the rear of the Home signal, and a Shunting Limit Board appears at the adequate distance in advance of the Outer signal (unless the advanced starter for the other direction appears there, which can be used as the shunting limit marker).
The arrangement of a ‘B’ class station allows two trains to be received simultaneously from either direction without block overlap or signal overlap infringement by either. The two trains must be received on the two loop lines. If one train must be received on the main line, then it is accepted directly and the other train is held at the outer signal by keeping it at danger. ‘B’ stations therefore have higher capacity than ‘A’ stations, as trains can be on on the main and loop lines simultaneously, while other trains can be waiting at either end on the block sections. ‘B’ stations are generally used for most single lines, and also for some double lines (except for suburban stations which for the most part use other arrangements with automatic signalling to increase capacity).
In MLQ signalling, a Distant signal is provided at an adequate distance from the Home signal; the Home is actually a combination Home and Warner signal or a bracketed home signal with a combination Home and Warner signal for the main line and additional home signals for the loop line(s). When all signals on the bracketed home are on, then the train must come to a halt and not proceed. For loop reception, the main home signal and warner are both on, and the loop home is taken off; the train is expected to proceed at 15km/h on to the loop and stop on the loop. If the main home signal is taken off while the warner is on (with the loop home being on, obviously), the train is expected to proceed at 15km/h on to the main line and stop there. If both the main home signal and its warner are taken off, the train is to run through on the main line.
Under the MAUQ / MACL systems, trains are received as follows on double lines. For reception on the loop line: Distant at attention, Home at Caution for the loop. For reception on the main line, Distant is taken off, Home is at Caution for the main line. For run-through, the Distant and Home for the main line are both taken off.
In 2-aspect territory, a Warner and a Home signal are provided. In other systems, a Distant and a Home are provided. The Warner or Distant must be at the braking distance from the Home signal, and should be controlled through block instruments. There is no starter signal, so a train can be received only after the previous train has passed an adequate distance measured from the home signal.
‘C’ stations usually exist only on double lines, as they provide no crossing facilities.
No fixed signals need be provided, and the train is stopped for discharging or picking up passengers under any ad hoc arrangement that is suitable
Unmanned Intermediate Block Posts
The signals for an unmanned intermediate block post are controlled by the station to its rear. Track circuiting is used to ensure that the last train has passed an adequate distance beyond the Home signal of the unmanned IBP before the next train is received.
In automatic block working, manually operated Home and Starter signals are provided at a block station. Minimally, an automatic stop signal is also provided to the rear of the Home signal. Additional automatic stop signals may be provided between any two block stations.
Continue to the section on aspects and indications of subsidiary signals. Or see more on interlocking and block system working.