Automatic Block Signaling:

In Automatic Block Signaling (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 km 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 signaling, when the train is 2 blocks ahead of the aspect then changes to Attention, and then to Proceed when the train has passed 3 blocks ahead. In 3-aspect signaling, the aspect changes to Proceed when the train is 2 block sections ahead. In automatic block territory, 2-aspect signaling is not used.

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 stations that come under the TMS (Train Management System) centralized traffic control system).


Automatic Block Signaling 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 signaling 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.

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 the 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.

Description of Panel:
  • A control panel is provided at every station of automatic signaling section.
  • Each station controls the movements of trains in one adjacent section.
  • Two illuminated arrows for each direction. When a direction of traffic is established, arrow gets illuminated White when the line is clear.
  • It changes to Red when block section is occupied or due to the failure of track circuit / Axle counter.
  • The signal normal indication to prove that signals of that side of the station are at ON.
  • Permission from (green indication) to indicate pressing of Push button at the controlling section.
  • Direction switch to establish the direction of traffic.
  • SMs lock up key – to lock up the panel in the last operated position to prevent unauthorized or inadvertent operation.
  • Emergency push buttons to permit change of direction of traffic in case of failure of track circuit/axle counter.
  • Counters (digital) to register each operation of emergency switches.