Concept of Overlap and Isolation:- Every stop signal by its indication to the driver controls the movement of train upto the next stop signal as the next stop signal will control the movement beyond it. Hence the track between the stop signal and the next has to be clear and the points have to be correctly set and locked before a movement is permitted by it. However due to any unforeseen reasons like with sudden brake inadequacy the driver may not be able to stop at the next stop signal.

So an extra safety margin of the track beyond the next stop signal is also to be kept free so that if the train overshoots the next signal, he will be able to bring the train to stop within that margin. This safety margin is called “overlap”. Similarly we have to ensure that when a train moves on the track the other rail vehicles from the adjoining track should not roll down and infringe with the movement. To prevent this “isolation” between adjoining lines is required.

Overlaps
Overlaps are referred to as ADEQUATE distance. Overlaps are of two types:
1) Block Over Lap (BOL)

2) Signal Over Lap (SOL)

Block over lap : It is the extra length of track in advance of the FSS (First Stop Signal) of a station, which must be kept clear, before Line clear can be given to the station in rear.

BOL in MACLS sections shall not be less than 180 Mts.

Signal over lap : The length of track in advance of a stop signal of station, which must be kept clear, before the signal next in rear could be taken „OFF‟.

SOL in MACLS sections shall not be less than 120 Mts.

Incase of Automatic signals the adequate distance is 120 Mts. minimum.

Concept of Overlap and Isolation | Overlaps | Block Over Lap (BOL) | Signal Over Lap (SOL) | SOL | Signal over lap | Block over lap

Gate Signal :- 

The Gate stop signal shall be provided with „G‟ marker. Letter „G‟ in black on a yellow circular disc.
A semi-automatic stop signal interlocked with a level-crossing gate shall be provided with „G‟ marker disc and an illuminated „A‟ marker. The „A‟ marker shall be lit only when the gates are closed and locked against road traffic.

Routing Indicator

Where two are more lines diverge, information is to be given to driver that he is being received on diverge line. Hence route indicators are provided.
Route indicators are fixed on the first stop signal and starters.
If the route indicator on a signal is not in working order, the relevant signal shall also to be treated as defective signal.
Route indicator is denoted as (UG).
Route indicator are of three types.

Junction type route indicator

Used where the speed is above 15KMPH
It is having a provision of indicating six diversions and a straight line.
When taken off it shows a row of five white lines.

Multi lamp route indicator

Used where the speed is less than 15 KMPH.
It can exhibit nine numerals and alphabets.

Stencil type route indicator

Normally fixed on starter signal.

Dead approach locking: It is seen that for providing approach locking, a track circuit for a length of 1,2kms. Need to be provided. Provision of such a long track circuit for the purpose of approach locking is a costly proposition. Therefore, the approach locking is provided without the approach locking becoming effective the moment the signal is cleared irrespective of the position of the train in the approach.

Such an approach locking is known as “Dead approach locking”. In other words, once the signal is „OFF‟ it gets approach locked and the route can be released (after putting back the signal to danger) only after a time delay of 2 minutes.

Back locking or route locking:

It is an electrical locking effective when a train has passed the signal and adopted to prevent releasing of the route while the train is “within the limits of the route entered”. For any signal, the route is from the foot of the signal up to the next signal ahead.

But, the limits of the route are from the foot of the signal up to and including the last point in the route. for example, for the home signal or the calling-on signal below it, the limits of the route for the purpose of back locking is from the signal up to the berthing track. The berthing track circuit shall not be included in the back locking.

For S10 and C10, the back locking track circuits are 10T, 10T1, 56BT and 54T for both the routes 10A and 10B. So long as the train is moving over these track circuits the back locking will be effective and route cannot be released. After the train clears 54T the route will be released automatically.

For S3 and shunt 103, the back locking track circuits are 3T and 56AT.

Isolation

The term isolation denotes the condition in which line for a particular movement of a train is separated from all adjoining lines connected to it in such a manner that it cannot be fouled or interfered with by any movement taking place on the adjoining lines .

Rules regarding isolation:

* A line, on which train movements at speeds higher than 50 KMPH are permitted, should be isolated from all connected lines.
* Passenger lines should be isolated from all connected goods lines and sidings, whatever the speed may be.
* Isolation of goods reception lines from sidings is considered desirable.
* To maintain safety in through running, points and trap sidings should be not be inserted in the main or through line except with the permission of CRS.
* Where other means cannot be adopted to permit simultaneous reception on a single line.
* To trap vehicles running away from a station.
* To avoid trains entering from block section due to heavy falling gradient.

Methods of Isolation :

1) Connecting to another line or a siding.

Railway Isolation Rules regarding isolation | Methods of Isolation

Fig 4.1 shows the Point Control and Lever Lock circuits for a set of double-ended flat bottom four foot points. The main ‘W’ fuse has been replaced by a Miniature Circuit Breaker (MCB). The 12V D.C style F3 LPR has been replaced by a 100V A.C QNHX1 plug in style ‘Q’ relay.

As this relay is AC, it is wired in parallel with the input of the lock rectifier. More recent ‘V’ style installations have been fitted with LKEs (red LED units) mounted in a panel to the rear of the frame. These LED units have an integral voltage limiting resistor and are wired in parallel with the coil of the lever lock.

Detection Circuits

Fig 4.2 shows the Detection circuits for a set of double ended flat bottom four foot points. The design of the WL Control circuit (which we shall look at next) requires that the points (P&LD) must be detected separately from the WL.  The three position WKR has been replaced by two QNHX1 line relays, which detect the points Normal (NWKR) or Reverse (RWKR).

When the points are neither Normal nor Reverse, both relays will be de-energised (equivalent to the mid position on a three position WKR) Because the polarity of the incoming feed no longer needs to be swapped over to drive the detection relays, the loops in the P&LD box have been removed and only the BX supply is switched.

Therefore only two contacts of the P&LD box are used. When the NWKR is energised and the WLs are in their ports, the NWLKR picks up. When the RWKR is energised and the WLs are in their ports, the RWLKR picks up. The NWLKR or RWLKR contacts are used in the signal selection circuitry of any signals that route a train over these points.