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,...
more... 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.
