Interlocking of Signals and Points


Interlocking is defined as the technique achieved through mechanical or electrical devices or agencies by which it can be ensured that before a signal is taken to “OFF” position, for the route, which the signal controls, is properly set and held, and at the same time all the signals and points, the operation of which may lead to conflicting movements, are locked against the feasibility of such conflicting movements.

Interlocking of Signals and Points


In signalling operations, it is absolutely necessary that only that signal should be lowered for which route it is meant and the route has been
advance. Interlocking is the device which helps in proper and safe working of the signals. With an increase in number of points and speed of the trains, it has become essential to eliminate the error on the part of human beings (i.e. cabin-man) and take all the precautions to prevent the possibilities of accidents. This is achieved by interlocking mechanism which makes the arrangements fool-proof.

Interlocking makes the arrangements fool-proof by performing following functions (i.e., Interlocking principles):
  1. It must be impossible to take ‘OFF’ a signal for approaching train unless the route (including the over-lap) to which the train is taking, is properly set, locked and held. This means that the points must be set and each facing point is locked so that it finds the passage of the train to withstand the stresses created by the train at the junction of divergence. At the same time, it is necessary that it must be impossible to operate the points (i.e., to unlock or reverse the points) while the train in moving on it; as otherwise, there is danger of the train taking conflicting routes.
  2. It must be impossible to take ‘OFF’ position at one and the same time for two fixed signals at the same time which would lead to conflicting movements (i.e., movements which cross each other’s path). So the points and signals should be interlocked against such movements.
  3. It must be impossible for loose wagons to interfere with the route for which the points are set and signal has been taken to ‘OFF’ position. For this purpose, it is necessary that levers operating points and signals should be interconnected in such a way that they can be pulled only in a particular sequence or order, and also can be put back in a particular sequence (i.e. reverse sequence or order).
  4. The route, for which the points are set and signal taken to ‘OFF” position, should be clear of any obstructions.


The functions (1), (2) and (3) are achieved by mechanical or electrical methods of interlocking of signals whereas function (4) can be achieved by track-circuiting which indicates the portion of the track is whether occupied or clear.


The mechanical method of interlocking consists of tappets and lock-bars. The tappet bars and lock-bars are placed at right angles to each other. The lock bars have extra metal piece fixed and cut to a shape such that it can be exactly fitted in the notches of the tappet bars.

Following figure shows the simple arrangement of only three levers which explains the principle of interlocking to prevent conflicting movements.

The principle of interlocking is as follows:
Tappet and Lock System
Tappet and Lock System
  1. The normal setting of the points is for main line, where the signal No. 1 indicates the routing signal for main line and signal No. 2 for the branch line (or turnout). The normal setting for the main line is for point No. 3 and signal No. 1 is up.
  2. To adjust the track for siding on branch line, the tappet bar for point No. 3 is pulled by lever No. 3 in the shown direction. This pushes the lock bar for point No. 3 out in the direction shown, which makes point No. 3 to be set for branch line.
  3. This pushing at the same time releases the lock ‘C’ from tappet bar for signal No. 2.
  4. Now, if the tappet bar for signal No. 2 is pulled through lever No. 2, it will prevent the lock bar for point No. 3 to track its path back and in this condition, it is said that signal No. 2 back locks point No. 3.
  5. At the same time, lock ‘C’ enters the notch of tappet bar for signal No. 1.
  6. So it is concluded that as long as signal No. 2 remains in ‘OFF’ position (i.e., lowered), point No. 3 would keep open for the branch line and signal No. 1 cannot be taken to ‘OFF’ position, (i.e., lowered).
  7. Such facilities for interlocking are provided for any number of signals to be operated in the station yard.


The mechanical devices are used mainly to meet the following purposes :

  1. A device to ensure that a route is properly set after which only the proper signal can be taken ‘OFF’ and the route cannot be changed after the signal has been taken ‘OFF’.
  2. Device to hold a route properly at a diverging point in spite of stresses caused by the train moving over it.
  3. A device to ensure that the route cannot be changed while the train is on the point even after putting back the signal.
  4. Point and signal levers are provided for locking and releasing one another in different positions so as to ensure correct routing, setting and avoiding conflicting movements.

To meet the above purposes the following mechanical devices are used :

  • Detectors
  • Stretcher Bar, Lock Plunger and Lock Bar
  • Tappet locking
  • Slotting of signals
  • Connecting devices such as rods, cranks and temperature compensators

(1) Detectors

It is one of the devices to ensure a mechanical relationship between the setting of points and taking ‘OFF’ of the corresponding signal. The detector is so called because it at once detects any defect or failure in the connection between switches and the lever or an obstruction between stock and tongue rail. The signal remains at danger position and cannot be taken to ‘OFF’ position until the defect is set right.

The detectors are used on all points over which signals control the train movements.

The mechanism of a mechanical detector in its simplest form is a box near the points wherein the bars from signal wires are placed at right angles to the bar from the stretcher bars of points. These bars are called slides. Signal slides are placed at right angles to the point slide and all the slides are suitably held, so that there is no vertical movement in them. The point slide
has a number of notches whereas the signal slide has only one notch.

The functions of mechanical detector are as below:
  1. When the points are set for the main route, the notch in the signal slide and the notch in the point slide, are face to face and the pull will be transmitted to the main home signal 3.
  2. If the points are reversed, the point slide will move to the left, the notch in the slide of signal ‘4’ and notch of the point slide will be face to face and signal 4′ will be taken ‘OFF’ (i.e., lowered).
  3. When signal “3” is taken ‘OFF’, signal “4” is locked and when signal “4” is taken ‘OFF’ signal ‘3’ is locked.
Points set for main line, signal slide free
(a) (b) Points set for main line, signal slide free

(2) Stretcher bar, lock plunger and lock bar

(i) Stretcher Bar

The only movable portions of the permanent way are the tongue rails at the location where two routes meet and due to stresses caused by a train at the divergence, this point is most vulnerable from the point of view of derailments. The points are known as weak links of the track. The two tongue rails are connected to each other by means of two stretchers, which are known as William patent stretchers. The front stretcher extends under the stock rail to prevent “jumping” at switches.

Detector Mechanism
Detector Mechanism. (c) Points set for siding of loop line

(ii) Point Lock

A point lock is placed in the middle of the track a little in front of the toes of tongue rail. The object of using a point lock is to ensure that each switch is correctly set. It may consist of one of the following types :

(a) When the speed does not exceed 16 km. p.h. a bolt and cotter is individually fitted to each switch rail and padlock or clamp, and a padlock for locking switch rail to stock rail.

(b) When the speed is more than 16 kmph but less than 48 km. p.h. a key of approved design for locking each rail independently.

(c) When the speed is more than 48 km.p.h. a plunger type of facing lock is used as shown in following figure –

Stretcher Bar, Lock Plunger, Point Lock and Lock Bar
Stretcher Bar, Lock Plunger, Point Lock and Lock Bar

The point lock consists of two stretcher blades, connected one to each tongue rail. These blades slide at right angles to the track as shown in above figure. The plunger moves in a plunger casing. The plunger is worked by a rod in the signal cabin is operated, the crank gives motion to both (i.e., lock bar and plunger) but the plunger moves at right angles to the stretcher blades. When the points are set for a particular route, the hole (or slot or notch) in the blade attached to one of tongues comes opposite the plunger rod inside the plunger casing and rod enters the slot, the point remains locked and it is impossible to move the switches.

(iii) Lock Bar

The lock bar is little longer than the longest where base of any vehicle. This is provided for the purpose that the point may not be operated while the train is on it. This is either of an angle iron or T-iron, 12.6 m minimum for B.G. and 12.0 m for M.G. (equal to longest wheel base), is provided near and parallel to the inner side of the rail. The lock bar is attached to the inside face of one of the rails by means of revolving clips.

At the time, when the rod of the point lock is worked from the signal cabin, the lock bar rises slightly above the rail level and then comes down. So, naturally when the train (or vehicle) is on the lock bar, it is impossible for the lock bar to rise above the level due to flange of the wheel. Thus the point cannot be operated when the train is on it.

(3) Tappet-Locking

This has already been described in above article.

(4) Slotting of Signals

Slotting is an arrangement in which though the lever operating a signal is in one cabin but the actual taking ‘OFF’ of the signal requires the releasing of a control by another cabin. Hence, a signal is to be operated and controlled from two different cabins. Besides this, the other requirement of slotting is that each of these cabins can unilaterally put back the signal to danger position. A simple example of slotting is between two non-block cabins at either end of a station say A and B.

In such a case, before one cabin man say A takes ‘OFF the Home signal for a train, he has to get an assurance that the line is clear for an adequate distance beyond the starter while that area is controlled by other cabin say B, and that from cabin B the information is obtained for setting up the points correctly. This is done through a slot from the other cabin. Following figure shows the cross section of a cabin.

Cross Section of a Cabin
Cross Section of a Cabin

(i) Electrical Slotting

This is of two types :
(a) Cabin-type Reversers.
(b) Post-type Reversers.

In Cabin-type Reversers, if the signal lever is pulled without obtaining the slot, the slotted side provided on the opposite side of connecting gear is pulled upwards and the beam swings about the opposite point as the fulcrum without pulling the signal arm. The slotted side has a notch and if the slot is given, an electrically operated block engages (or fits) in the notch and as the slide is fixed, the beam swings with the end of the slide as the fulcrum and thus signal wire gets pulled as shown in following figure.

Cabin type Reversers
Cabin type Reversers

In Post-type Reverser, the rod connecting the semaphore arm to the weight, has got two portions which get joined together by a wedge operated by the slot from the other cabin.

(ii) Mechanical Slotting

The mechanical slotting is also of two types :

(a) Three-way slot (3-lever slot), (b) Dis-engager

In 3-way slot, it has got three weights that are operated from separate
cabins. The signal arm is coupled to the balance weight. Two weights are on one side and controlled separately from the cabins; the balance weight being on the other side as shown in below figure.

The signal can go to ‘OFF position only when both the weights go up by both the cabins operating the levers. If on the other hand, any of the levers is put back in any of the cabins; the weight connected with it will go down putting the signal to danger i.e., ‘ON’ position.

In dis-engager, the connecting gear of the lever operating the signal has a break, so in this position, the operation of the lever does not move the signal arm. The connecting gear has a slide in the slot lever which gets engaged in the slide of the connecting wire of the signal lever. Thus a connection between the lever and signal is established. In reverse direction both the slots and signal lever will throw the signal to danger-position.

Three Level Slot Arrangement
Three Level Slot Arrangement

(5) Connecting Devices and Temperature Compensators

(i) Connecting Devices

The connecting devices between the lever in the signal cabin and switch (or signal). Usually consist of 3.8 cm diameter pipe or solid rod. At every change of direction, a suitable crank is used. These pipes move on rollers in frames fixed in concrete at 2 m to 3 m intervals. The distance between a cabin to points or signals (known as rodding signal) should not exceed 275 metres. There is no limit of this distance when points or signals are operated by power.

Connecting Devices
Connecting Devices

(ii) Temperature Compensators

Temperature Compensators are used to neutralise the effect of expansion or contraction of rods due to variations in temperature. All the double wire transmissions used for operating signal points, locks and detectors must be provided with compensators. These are provided at about 24 m distance or less in the line of rodding as shown in following figure. A compensator consists of a pair of cranks (one of acute angle and another of obtuse angle) connected by a pipe upto a wire length of about 365 m, only one compensator is used. But two compensators are installed if the length is more than 365 m.

Temperature Compensators
Temperature Compensators

It is desirable to put the compensators in the middle of the length if one compensator is used. The compensators should be placed at proper positions if they are two or more. For two compensators, they are used at quarter points.

The use of compensators at proper position is very essential. Because the compensator by virtue of its arrangement automatically compensates the variation in temperature.

As shown in above figure, whether point ‘C’ or ‘D’ may move to the left or right. But the points A and B will always remain at constant distance. The expansion or contraction of rodding or pipe lengths is adjusted within the compensator (i.e., between C and D).


With the increasing number of points, signals, etc. to meet the heavy and rapidly increasing traffic demands at high speeds. It is necessary that chances of conflicting movements arising out of human errors are eliminated in order to ensure safety of passengers and goods. This objective is achieved through interlocking of signals and points. So that only points desired for movement are open and all others are locked, till the operation is complete.

Interlocking plays a vital role in preventing conflicting movements and thus accidents, derailments, etc.

Sometimes, we observe defects in track, let us know how to fix them.

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