Single Line Block Instruments
by Apurva Bahadur
This article is based on text and line drawings found in an IR (SCR) signalling manual. Additional notes and illustrations added by IRFCA members. An earlier version of this article appears on Signalbox.org.
Types of Token Block Instruments
The SCR signalling manual includes the clause: "The term Station Master wherever used in this Manual, also applies to Assistant Station Masters, Cabin Assistant Station Masters, Switchmen and any other competent staff who may, for the time being, be in charge of Block Working."
Token-based working on single line sections relies on the possession of a token (either physically or with numbered vouchers, etc.) to designate authority for a train to enter a section. The token is relinquished when the train leaves the section. The block instruments are constructed in a manner so as not to allow more than one token to be released for the single line section at any time - to release one, any previously issued token must have been re-inserted into the system at either end, which serves as a guarantee that the previous train has left the section.
The types of Token Block Instruments in use on the Single Line sections of the Indian Railways are:
- Neale's Ball Token Instrument
- Neale's Voucher Block Instrument
- Neale's Tablet Token Instrumen
- Theobald's key Token Instrument
- Webb and Thompson Electric Train Staff Instrument
Note: Other kinds of token instruments have been used in India; this article only refers to those that were still in use in the late 20th century, and which, therefore, still found a place in the signalling manuals of IR.
Description of Neale's Ball Token Instrument
| (A) The token receiver cover | The token receiver cover should be lifted for inserting the token into the instrument |
| (B) Token Receiver handle | This should be turned token to fall into the instrument. |
| (C) Galvanometer | This indicates the flow of current from one instrument to another |
| (D) Telephone | This is provided in conjunction with the instrument for communication with the station at the other end of the block section. |
| (E) Plunger | This is used for transmission of Bell Code signals and operation of Operating Handle. |
| (F) Operating Handle. | .This can be set at Line Closed or Train Going To or Train Coming From positions. It can be turned to any one of these positions, when a prolonged beat is received from the station at the other end of the block section. |
| (G) Token Exit | A Token comes out through this exit when the Operating Handle is turned to the Train Going To position. |
| (H) Token Windows | This indicates the availability of tokens in the instrument. |
| (I) Token Indicator | The indicator shows a green disc if there is a token and a red disc if there if no token in the instrument. |
| (J) Bell | This responds to Bell Code signals given by the station at the other end of the block section. At stations provided with more than one instrument different bells or gongs with distinctive sounds are fitted up to identify the individual instrument. |
| (K) 'E' type lock with key in Train Going To position | This is provided where the Last Stop signal is controlled mechanically through the Block Instruments. The key can be released only when the block handle is in Train Going To position and is used for controlling the Last Stop signal lever. This key can also be used for controlling the slip siding points |
| (L) ' E ' type lock with key in Train Coming From Position. | This can be released only when the Operating Handle is in the Train Coming From position and is used for controlling the catch siding point |
| (M) Station Master's key. | When the key is taken out, it locks the instrument in the last operated position and, only incoming beats can be heard. This key should be kept in the personal custody of the Station Master when it is not required for operating the instrument. |
Note: The number of the token and the code initials of the two stations at each end of the block section to which the token applies are engraved on each token. Tokens are provided with grooves of a different pattern for different sections. Ball tokens are provided in 4 different classes viz. A class, B class, C class and D class for different block sections.
Description of Neale's Voucher Block Instrument
| (A) Aperture for inserting tokens | Through the aperture slot a ball token is put into the Block instrument. |
| (B) Galvanometer | This detects the flow of current from one instrument to another. |
| (C) Telephone | This is provided in conjunction with the instrument for communication with the station at the other end of the block section. |
| (D) Bell plunger. | This is used for the transmission of Bell Code signals and operation of Operating Handle. |
| (E) Operating Handle. | This has three positions, Line Closed, Train Going To, and Train Coming From. It can be turned to any one of these positions when a prolonged beat is received from the station at the other end of the block section. Note: When a token is dropped in, the Operating Handle can be brought to Line Closed position without the cooperation of the Station Master at the other end of the block section. |
| (F) Token Delivery cup | A token comes out through the token delivery cup |
| (G) Bell | This corresponds to the Bell Code signals given by the station at the other end of the block section. At stations provided with, more than one instrument different bells or gongs with distinctive sounds are fitted up to identify the individual instrument. |
| (H) Station Master's key | When the key is taken out, it locks the instrument in, the last operated position. This key should be kept in the personal custody of the Station Master when it is not required for operating the instrument. |
| (I) Switches for controlling locks. | The left-hand switch controls the token lock and shall be pressed while extracting the token. The right-hand switch shall be used when turning the handle toTrain Coming From or clearing back or canceling. |
| (J) 'E' type lock with key in Train Going To position. | This is provided where the Last Stop signal is controlled mechanically through the Block Instruments. The key can be released only when the block handle is in Train Going To position and is used for controlling the Last Stop signal lever. This key can also be used for controlling the slip siding points. |
| (K) 'E' type lock with key in Train Coming From position. | This can be released only when the Operating Handle is in the Train Coming From position and is used for controlling the catch siding points. |
Note: The number of the token and the code initials of the two stations at each end of the block section to which the token applies are engraved on each token. Tokens are provided with grooves of 3 different patterns for different sections viz. broad groove, narrow groove and double narrow groove.
Description of Neale's Tablet Token instrument
The instrument is similar to Neale's Ball Token instrument except for the following differences:
Token slide with Handle: A token slide is provided at the top of the instrument instead of a Token Receiver cover. A tablet token is inserted into the instrument by pulling out the slide, placing the tablet token in the recess and pushing back the slide.
The Neale's Tablet Token Instruments are provided with tablet windows instead of token indicators.
Description of Theobald's key instrument
| (A) Key Token | It consists of a metal key with lugs at both ends and a tongue at the center; the lugs suit the key guards of the instrument. Each token is engraved with the number of the token. |
| (B) Key guards | The Key Guards are small metal brackets provided on the face of the instrument on either side to receive a token of a definite class. |
| (C) Galvanometer | This detects the flow of current from one instrument to another |
| (D) Plunger | This is used, for giving Bell Code signals and permitting extraction of a token. |
| (E) Bell | This responds to the Bell Code signals given by the station at the other end of the block section. At stations provided with more than one instrument different bells or gongs with distinctive sounds are fitted up to identify the individual instrument. |
| (F) Locking bolt and padlock | The locking bolt should be fitted in the slot for preventing token being taken out, It should be padlocked when it is not in use and the key is kept in the personal custody of the Station Master to. prevent unauthorised manipulation of the instrument. |
| (G) Token Magazine | Key tokens are stored in the staff magazine. |
Description of Webb & Thompson Instruments
| (A) Indicator | Needle/Dial indicating Staff In and Staff Out In case of 'M' type instrument the indicator needle indicates Staff In and Staff Out and in the case of 'S' type instrument the dial indicator indicates Staff In and Staff Out. |
| (B) Galvanometer Needle | The Galvanometer Needle indicates whether the station at the other end of the block section is authorising extraction of Staff or that the Staff has been extracted. |
| (C) Cut out switch knob | This should be pressed after extraction of the Staff from the station at the other end of the block section that the Staff has been extracted |
| (D) Brass Thumb Plate | |
| (E) Staff inlet and outlet | Through this, the Staffs are inserted into or withdrawn from the instrument. |
| (F) Staff Magazine | The Staffs are stored in the Staff Magazine |
| (G) Train staff | The Train Staff token is an iron bar about 300 mm long with a brass cap at one end about 50 mm in length on which are inscribed the number of the staff and also code initials of the stations at each end of the block section to which it applies. The Train Staff is provided with rings of different patterns for different sections. |
| (H) Generator | The handle of the Generator should be rotated in clock wise direction generating the current for transmission of bell beats and extraction if the Staff token at the other end by pressing the Key plunger pertaining to the block section |
| (I) Key plungers | They are provided for signaling Bell Codes to the station on either side by depressing the appropriate Key plunger and the Generator handle being turned. |
| (J) Bell | This responds to the Bell Code signals given by the station at the other end of the block section. At stations provided with more than one instrument different bells or gongs with distinctive sounds are fitted up to identify the individual instrument. |
| (K) Block Telephone | This is provided in conjunction with the instrument for communication with the station at the other end of the block section |
Indoor Apparatus
The control on the last stop signal may be:
(1) For Neale's Ball and Neale's Tablet and Voucher Block Instruments:
- By using 4 mechanical control key extracted from the instrument adjunct on receipt of Line Clear to release the Last Stop signal, or
- By electrical control on reverser being released when Operating Handle is turned to Train Going To position, or
- By electrical lever lock on route lever for dispatch being released when Operating Handle is turned to Train Going To position.
Note: There is no control on Last Stop signal through the Block Instrument where Webb & Thompson Instruments are provided.
(2) Theobald's key Token instrument:
- By means of Station Master's control slide which should be pulled before extracting a token.
Note: Where Starters or Advanced Starters are not provided in the Upper Quadrant territory, the Proceed aspect of the Main Home signal should be controlled through the Block Instrument and in Lower Quadrant territory, the Proceed aspect of Main line Home signal should be controlled through the Block Instruments.
Block Line will be cut if extraction of token is not done carefully. If this happens, the Station Master's control slide should be normalised and reversed again and extraction of token should be done thereafter.
Outdoor Apparatus
An electric signal reverser is provided in most of the cases for controlling the Last Stop signal.