How do the block or token instruments work?

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How do the block or token instruments work?

July 16, 2019, 12:38 PM

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.

Source – IFRCA.org

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