The Loco pilot of the train is very much dependent on the visibility of the signals. Some time is taken by any human being between seeing or hearing the certain thing and reacting to the same. This time between seeing or hearing and reacting to the same is known as reaction time. So is the case with the Loco pilot. This time is taken to be seven seconds. A train moving at a speed of 100kmph shall traverse 194 meters in seven seconds. The second major factor to decide minimum visibility of the signal is the braking distance, which in turn depends on the braking power, the total mass of the train, gradient and the speed of the train. Taking into consideration all of these factors the minimum visibility of the signals has been prescribed to be as follows:

Multiple aspect signaling systems:

Distant signal- 400 meters and if inner distant is also provided the visibility for inner distant shall also be 400 meters.All other signals at the station- Aspect of each signal shall be visible from its previous signal in the direction of the train.Where adequate visibility of the stop signal cannot be maintained, repeater or co-acting signal shall be provided or speed restriction shall be imposed if the desired level of visibility is not available.If no signal indication is available to the Loco pilot, he shall control the train considering that the next signal is in danger.

Shunt signal:

This is meant for signaled shunting in the interlocked station section performed with a locomotive. The shunt signal leads movement from one shunt signal to the next shunt or stops signal whichever falls first on the route. When a shunt signal is mounted on the same post as that of a running signal, both the main or shunt signal cannot be taken off at the same time. Either the main signal shall be taken off or the shunt signal at any one time.

Calling on signal:

In bigger yards where reception signal is placed at such a distance and on a route with high density traffic where loss of time to the train is with serious consequences resulting in queuing up of following trains, the Calling on signal is provided on the same post as that of reception signal below the main signal arm or unit to receive the train when the main running signal has failed or the train has to be received on an occupied line. The Calling on signal and main signal on the post of which the calling on signal has been provided, cannot be taken off simultaneously. The calling on signal may also be provided on the post of signals other than the reception signals, under approved special instructions except on the post of last stop signal.

Route holding:

After the signal has been taken off it shall not be possible to move any point or lock on the route, including overlap and isolation until the signal is replaced to the ON position. Route as mentioned here covers all points, locks and level crossing gates on the track, in between signal and the destination as well as the overlap that is the specified distance of 180 meters in case of two aspect signaling and 120 meters in case of multi-aspect signaling, beyond the signal next to the signal for which route holding is considered here.

When a train is approaching the station from a Block section and the Loco pilot sees the signal approaching as OFF, the route to which the signal leads should get locked and it shall not be possible to be disturbed. It is possible only when there is a track circuit available between the sighting board and the signal which when occupied shall detect the arrival of the train within normal braking distance. It shall also not be possible to disturb the route even when the train has passed the signal and even it is replaced to ON, till the train has arrived complete.

Approach Locking:

Holding the route beyond the signal, to which it leads, while the train is approaching, is called Approach locking. When the train is approaching a signal and the Loco pilot has seen the signal OFF, which means accepted the signal as Off, he shall get busy in taking action with reference to the aspect of the signal. In case the aspect was green he shall further get relaxed and continue to move at optimum speed, but if the signal is Off with caution or attention aspect the Loco pilot shall be further busy in controlling the speed of the train depending upon the aspect of the signal. By the time he gets relaxed he must have traversed the considerable distance.

During the intervening period at a speed of 60 kmph, he shall have traversed one km in one minute which means if the Distant signal is at one km from shoe he shall be almost crossing the home signal. Putting back the signal in order to change the route at the last moment when the loco pilot has accepted the Off aspect, shall be extremely dangerous if the train encounters an unlocked or moving point under the wheels. The approach locking can be provided only when the track between the signal, being approached, and the sighting board, when the train is approaching the station from the block section, is track circuited.

Generally, this is available only when Automatic Block signaling is provided in the adjoining block section where track circuits are available by default. However, at stations provided with panel interlocking, entire yard is track circuited as such approach locking for all other signals in the yard is available except for Home signal and for Shunt signals provided on the siding which are not track circuited to the entire length of the siding.

When a track circuit has been provided between the Sighting Board and the signal to which the Loco Pilot is approaching from the Block section, the route gets approach locked as soon as the track circuit is occupied by the approaching train. The route shall not be Approach locked till the train occupies the track circuit and in such situation, it shall be possible to change the route. In such a situation the route shall be released immediately upon putting back the signal. After the Loco Pilot has accepted the signal, the route shall be released when the train reaches its destination, in the normal course.

Dead Approach Locking:

Approach locking is known as ‘Dead Approach locking’ when the signal, once taken ‘Off’, locks the route dead and the route cannot be released upon putting the signal back to ‘On’ except after a lapse of 120 seconds after the signal is put back to “On’, since there are no means to hold the route such as a track circuit at a place where the Loco Pilot accepts the signal. The Dead Approach Locking is required to be provided for routes of signals leading from non-track circuited approach track, which usually are Home signals and Shunt signals.

Back Locking:

After the train has passed the signal, it is important to put back the signal to ‘On’ which in turn shall release entire route ahead, which was kept locked by the signal level or approach locking circuits so far. When a track circuit is provided ahead of the signal, the signal is replaced back by the occupation of the track circuit. In case of mechanical signal the reverser provided with the signal is released putting back the signal to ‘On’ and in case of the color light signal, the signal is put back to ‘On’ by the occupation of track circuit through selection circuits. Occupation of this track circuit also simultaneously initiates back locking circuits.

Track Locking:

It is holding the points locked while it is under the wheels of a train. Mechanically it is done by attaching a lock bar to the facing point lock. The length of the lock bar is kept as such to cover the maximum distance between the two successive wheel bas3s of any vehicle. The length of lock bar is accordingly dept to be 42 feet. The lock barf attached to the facing point lock plunger moves to-and-fro along with the plunger through a radial guide, due to which the lock bar has to lift up to rail top level.

The unlocking movement of the plunger is not possible if a wheel is over the lock bar as the flange of the wheel shall not allow the lock bar to be lifted to the level of rail top. Electrically, the track locking is also provided through track circuits in the entire point zone. The occupation of the track circuit detects the presence of a train over the point zone, which keeps the point locked through selection circuits.

Flank Protection:

The other area of safety which is required to be monitored by the system is protection against side collisions. Two railway lines running parallel to each other are kept at a specified minimum distance between them. If the given distance between the two tracks is reduced, the trains running on such tracks shall infringe with each other and shall result in the side collision. Whenever two tracks join each other through a turnout or crossover, the two tracks infringe each other till the distance between the two has arrived at the safe range.

The spot which the two tracks become unsafe is known as fouling mark. The protection for trains on account of the infringement of tracks is called flank protection. When the flank protection is achieved through track circuits, the track circuits are so designed that unless the train stands clear of the track circuit, it shall not be possible to permit the train movement on such track. In the yard where point zone is not track circuited the flank protection is achieved by using fouling or clearance bars. A 42 feet uneven angled MS bar known as fouling bar or clearance bar depending its place of usage, is coupled with the point in such a way that it shall not be possible to operate the point till the Fouling or the Clearance bar is kept pressed under the wheel.

The point for the adjoining line can be set only when the fouling or clearance bar is cleared by the train entering the birthing part of the track.The fouling bar is linked with the trailing point in such a way that the point cannot be operated from its last operating position unless the last train received on the main line clears it. Similarly, the clearance bar is attached to the trailing point in such a way that it shall not be possible to operate the point unless the last train received on loop line clears the bar.