Page 4

The concept of block overlap and signal overlap was more relevant in older signaling environment as in those earlier times when the technology was not well developed different authorities controlled different stations and communication facility between them was limited. This meant there was a more chances that a train may be dispatched  from a  station not aware of  obstruction existing at the next station being approached than when a signal lead to a signal within the control of the same authority.

In fig 3 the signal S5 is placed an adequate distance in rear of the point P1  so as to ensure that in the unlikely event of a train overshooting signal S5 the point will not be infringed thus saving the train from being derailed due to a point not being in the proper position to allow trains to run over it. Distance between Point P1 and S5 should be more than the block overlap.

 With the improved highly reliable telecommunication infrastructure to support the Railway signaling the separate concept of Block overlap and signal overlap is significantly diluted. In fact what is more relevant is the assessment of actual overlap requirement based on the speed of the trains and its braking capabilities. In the present scenario the use of additional signals to improve safety under this condition is made available and strictly speaking the only overlap which needs to be considered is the signal overlap based primarily on the braking distance.

After the first stop signal other signals are provided to ensure movement in the station area as required. signals are required to ensure that all diversions are protected properly. This means that if there is a diversion protection is available. Signal S5 in figure 3 protects the diversion provided at point P1.

Protection while a train negotiates a diversion can be ensured by two methods

a) By a signal placed just ahead of the diversion within say not more than 180m typically.

b) By locking the switches allowing the diversion till movement leading from the signal leading to the diversion and in this condition the signal can be located any distance before the diversion or even before other diversions

Both methods are actually same and is done by locking the movement of the switches till train signaled to move over the point has passed over it. But in terms of signaling the two are slightly different due to the fact 

In case a) the signal is very near to the switch and so when the signal is cleared the train is over the point almost immediately and simply by ensuring that the point zone track circuit is dropped it can be ensured that the switch cannot be moved. This is covered b what is referred to as Approach locking.

In case b) however the signal leading to the diversion can be considerable distance away and so will take considerable time to hit the point zone track circuit. Thus another more elaborate locking is  involved which proves sequential operation of tack circuits to determine whether a train which has been signaled has completed movement over the diversion and till such time it is completed the locking of the  switch needs to be maintained. This is referred to as Route Locking.

If looked at from a fundamental point of view there is very little difference between a) and b) and b) is actually an extension of a).