Railway Interlocking

Interlocking

Interlocking is a device or a system meant to ensure the safety of trains. With the increase in the number of points and the signals and introduction of high speeds, it has become necessary to eliminate human error, which would otherwise lead to massive losses of life and property. The points and signals are set in such a way that the cabin man cannot lower the signal for the reception of a train unless the corresponding points have been set and locked. The signal is thus interlocked with the points in a way that no conflicting movement is possible and the safety of trains is ensured.

Interlocking can, therefore, be defined as an arrangement of signals, points, and other apparatus so interconnected by means of mechanical or electrical locking that they can be operated in a predetermined sequence to ensure that there is no conflicting movement of signals and points and trains run safely.

The signal and interlocking system is so designed that the failure of any equipment results in the turning on of the signal, thus ensuring train safety.

1 Essentials

Lever frames and other apparatus provided for the operation and control of signals, points, etc., must be so interlocked and arranged as to comply with the following essential regulations.

(a)  It should not be possible to turn a signal off unless all points for the line on which the train is to be received are correctly set, all the facing points are locked, and all interlocked level crossings are closed and inaccessible to road traffic.

(b) The line should be fully isolated before the signal is turned off, i.e., no loose wagons should be able to enter this line.

(c)  After the signal has been turned off, it should not be possible to make adjustments in the points or locks on the route, including those in the isolated line. Also, no interlocked gates should be released until the signal is replaced in the 'on' position.

(d) It should not be possible to turn any two signals off at the same time, as this can lead to conflicting movements of the trains.

(e)  Wherever feasible, the points should be so interlocked as to avoid any conflicting movement.

2 Standards

The speed of a train depends on a number of factors such as the haulage capacity of the locomotive, the fitness of the track, the fitness of the rolling stock, the load of the train, etc., and the speed for a particular section is determined based on all these factors. Depending upon the maximum speeds permitted in a section, the stations are interlocked in keeping with the prevalent standards, and signalling equipment and other facilities are provided accordingly. There are four standards of interlocking based on the maximum permissible speeds prevailing on Indian Railways. These refer to the speeds over the main line with respect to the facing points and the yard.

Table 31.11 lists the stipulations laid down by Indian Railways for signals, isolation, points, and interlocking arrangements. The details of each of these interlocking standards are enumerated in Table 31.12.

Table 31.11   IR's stipulations for interlocking

3 Methods

There are basically two methods of interlocking as explained below.

Key interlocking

Key interlocking is the simplest method of interlocking and still exists on branch lines of small stations on Indian Railways. The method involves the manipulation of keys in one form or the other. This type of interlocking is normally provided with standard I interlocking with a speed limit below 50 km/h. The simplest arrangement of key interlocking is accomplished in the following manner.

(a)  Take the example of a station with a main line and a branch line. The point can be set either for the main line or branch line.

(b) The point has two keys. The first is key A, which can be taken out when the point is set and locked for the main line. Similarly, key B can be taken out when the point is set and locked for the loop line. At any given time either key A or key B can be taken out, depending upon whether the route is set for the main line or the loop line.

(c)  The lever frame operating the signals is provided with two levers. The lever concerning the main line signal can be operated only by key A and similarly the branch line signal lever can be operated only by key B.

(d) If the train is to be received on the main line, the points are set and locked for the main line and key A is released. This key is used for unlocking the main line signal lever, thus lowering the signal for the main line. Since key A cannot be used for interlocking and lowering the branch line signal, only the appropriate signal can be turned off. This type of interlocking is called

indirect locking.

In case more than one point is to be operated, the key released at the first point is used to unlock and operate the second point and so on. The key released at the last point can then be used for unlocking the lever operating the appropriate signal. This type of interlocking is also known as succession locking and is also used for checking conflicting movements in shunting operations. There are other methods of interlocking with the help of keys, but all of them involve considerably lengthy trips from the point to the signal levers and from point to point, thereby leading to delays. Such arrangements are, therefore, satisfactory only for stations that handle very light traffic.

Table 31.12   Details of the different standards of interlocking

Mechanical interlocking

Mechanical interlocking or interlocking on lever frames is an improved form of interlocking compared to key locking. It provides greater safety and requires less manpower for its operation. This method of interlocking is done using plungers and tie bars. The plungers are generally made of steel sections measuring 30 cm × 1.6 cm and have notches in them. The tie bars are placed at right angles to the plungers and are provided with suitably shaped and riveted pieces of cast iron or steel that fit exactly in the notches of the tappets.

The main components of an interlocking system are a locking frame, point fittings, signal fittings, and connecting devices for connecting the locking frame to the point and signal fittings. The locking frame consists of a number of levers, which work various points, point locks, signal levers, etc. The levers are arranged together in a row in a frame. Pulling a point lever operates the point to which it is connected through a steel rod. Similarly, pulling a signal lever changes the indication of the signal by pulling the wire connecting the lever and the signal. To each lever is attached a plunger which has suitably shaped notches to accommodate the locking tappets. The entire arrangement is provided in a locking trough where tappets are provided, which move at right angles to the plungers.

When a lever is pulled, it causes the plunger to which it is connected to move. Due to wedge action, the tappet accommodated in the notch of the plunger is pushed out at right angles to the movement of the plunger. The motion is transmitted to all other tappets that are connected to this tappet through a tie bar. As a result of this motion, the other tappets either get pushed into or out of the respective notches of the other plunger depending upon the type of interlocking provided. In case the other tappet is free but slips inside the notch of the other plunger, it locks the lever connected to this plunger. In consequence, the other lever gets locked in that position and cannot be operated. However, if the tappet was earlier positioned in the notch of the plunger, thereby locking the lever, and is now out of the notch, the other lever becomes free to be operated.

4 Different Cases

The following cases of interlocking are encountered in practical application.

Normal locking In this case, pulling one lever locks the other lever in its normal position.

Back locking or release locking In this case, when the lever is in its normal position, it also blocks the other lever in its normal position, but when this lever is pulled it releases the other lever, which can then also be pulled. Furthermore, once the second lever is also pulled, the first lever gets locked in the 'pulled' position and cannot be returned to its normal position unless the second lever is restored to its normal position.

Both wall locking In this case, once a lever is pulled, it locks the other lever in its current position, i.e., in the normal or pulled position.

Special or conditional locking In this case, the pulling of one lever locks the other lever only when certain conditions are fulfilled, say the third lever being in a normal or pulled position as the case may be.

5 Mechanical Interlocking of Points and Signals of a Two-line Railway Station

To understand the mechanical interlocking of the points and signals of a two-line railway station, let us take the case of a typical railway station that has both a main line and a loop line. It is provided with a home signal 1 operated by lever 1 for the main line and a home signal 2 operated by lever 2. Point 3 is set for the main line when lever 3 is in its normal position and for the loop line when lever 3 is pulled. In its pulled position lever 4 locks point 3 in either position (normal or reverse) by pushing the plunger of the facing point into the 'lock in' position. The essentials of this interlocking system are as follows (Fig. 31.21).

(a)  It should not be possible to turn off both the signals, i.e., 1 and 2 at the same time, i.e., the train should be received either on the main line or the loop line at any given time. To achieve this, tappet A is forced out of the plunger of lever 1 when lever 1 is pulled. The tappet enters the notch of the plunger of lever 2 and cannot move out until lever 1 is pulled. This prevents lever 2 from getting pulled when lever 1 is pulled. The reverse of this situation is also true.

(b) It should not be possible to turn a signal off until and unless the point is set and locked. If an effort is made to pull either lever 1 or 2, the same cannot be pulled. This is because of the following reasons. Tappets B and C are rivetted with a tie bar that is in turn connected with tappet D. Since tappet D is butting against the face of the plunger of lever 4, tappet B or C cannot be moved. This plunger can move only when lever 4 is pulled, thereby bringing the notch of the plunger opposite tappet D. Once this happens, it is not possible to restore the position of lever 4 till lever 1 or 2 has been brought back to its normal position. This is a case of release locking. Levers 1 and 2 are released by pulling lever 4, which in turn locks the point.

(c)  Lever 4 is a lock lever and locks point 3 in either position. The same can be visualized for tappet E.

(d) Similarly, the main line home signal lever 1 cannot be pulled if point lever 3 is pulled, i.e., if it is set for the loop line, as the tappet F cannot be pushed, which allows lever 1 to be pulled when the notch in the plunger of lever 3 that lies opposite tappet F has been shifted from its position due to the pulling of lever 3. With lever 3 normal, lever 1 can be pulled conveniently, simultaneously resulting in the locking of lever 2 in it normal position by

tappet A.

This example indicates that, with proper planning, it is possible to mechanically interlock the movement of points and signals and thus ensure complete safety.

6    Electrical Interlocking of Points and Signals of a Two-line Railway Station

Electrical interlocking is achieved through electric switches known as relays. The manipulation of relays achieves interlocking, whereas lever locks that are attached with the levers in place of plungers or in addition to plungers prevent a lever from getting pulled, or allow it to get pulled or normalized if the interlocking so permits.

Relays use the simple principle of electromagnetism, whereby a soft iron core wrapped inside a wire coil turns into electromagnet when current is passed through the wire. An armature is attached to this electromagnet, which has a number of finger contacts that come into contact with each other when the armature is attracted to the magnet and break the contact when the armature is not attracted to it. The whole system is housed in a glass or metal box and is known as relay.