What’s ‘interlocking’?

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July 16, 2019, 12:12 PM

In order to ensure that the signalling system never provides unsafe (conflicting) signals and the points are not set for more than one train that might end up proceeding on to the same section of track and hence suffering a collision, various schemes have been developed to coordinate the settings of the points and the signals within the region controlled by a signalbox or signal cabin.

Mechanically operated interlocking

The most prevalent systems today [2003] are still mechanical interlocking schemes that coordinate the positions of the levers controlling the points with the signals governing that section of track and connected branches, loops, or sidings.

For instance, in one common scheme, a key that allows setting the points for a route has to be obtained from the block instrument, and as long as the key is removed the instrument cannot be set to provide Line Clear for a conflicting route. The wires that operate signals, and the rods that control points, are all interconnected in the lever frames at the signal cabins so that they are literally ‘interlocked’ — the position of one lever or key physically obstructs the movements of other levers and keys which control points or signals that can be set in conflicting ways.

Manually operated interlocking

This is a form of mechanical interlocking as well, but relies on the signalman to move about from one set of points and signals to another carrying with him the keys used to operate them. At small stations and on less busy branch lines various forms of manually operated mechanical interlocking are still [11/03] widespread. At points controlling catch sidings in hilly areas, often the interlocking is manual where the driver has to use a key provided by the stationmaster or signalman of the last station before the siding — the key is inserted into the interlock box which notifies the signal cabin and the points are then set for the main line and the signal is pulled off, giving the train authority to proceed. (This system is common in many hilly areas, although busier lines with catch sidings are being provided with automatically operating delayed signals where the points are controlled by a timer and are set to the main line only after the train has halted for the prescribed period of time.)

Even as of the late 1980s one had only to go a few kilometers outside any big city along a railway line to spot tiny signal cabins or block huts along quiet lines where all the action was done by the signalman unlocking and locking points and pulling off signals after talking on the magneto-telephone to his counterpart at the next signal ahead.

Hepper’s Key Transmitter Instruments and similar key dispensing instruments were in wide use for operating points manually. Depending on the state of the block instrument and the interlocking set up for the local layout of lines, these instruments would dispense a key only when the appropriate combination of signals had been set; the key would then be used to unlock points to divert a train on to a loop, for instance — and while the points were so set, that key would not be released and the signals could not be changed. Hepper’s Key Transmittters and other key interlocking systems (Annett’s Key, etc.) allowed for operating interlocked ground frames and points outside the range of direct mechanically connected operation.

A common system in use was Sequential Key Interlocking, which saved on the installation of point rodding and instead relied on the signalman walking over with a key to lock or unlock points. As an example, consider a station with a main line and a loop line. To receive a train on the main line, a key is inserted into the signal frame in the cabin or platform, which allows the Outer and Home signals of the station to be pulled off.

In order to receive a train on the loop line instead, the key is used as before to pull off the Outer signal, but the Home is kept at danger. Instead, when the train has stopped at the Home signal the key is removed and taken to the facing points for the loop. The same key unlocks the points so they can be set for the loop; it also releases another key which has to be taken back and inserted in the signal frame at the platform to pull off the Home signal to let the train advance on to the loop.

The mechanism was such that only one of these two keys could be released at once; the second key did not allow the operation of the Outer signal, and it had to be taken back to the facing points of the loop in order to release the first key.

Electrically operated interlocking

In the more advanced electrical or electronic interlocking schemes, the points and signals are worked from one integrated mechanism in a signal cabin which features a display of the entire track layout with indications of sections that are occupied, free, set for reception or dispatch, etc. The interlocking is accomplished not by mechanical devices but by electrical circuitry — relays and switches in older electrical or electropneumatic systems, and computerized circuits in the newer electronic systems.

Panel Interlocking (PI) is the system used in most medium-sized stations on IR. In this, the points and signals are worked by individual switches that control them. Route Relay Interlocking (RRI) is the system used in large and busy stations that have to handle high volumes of train movements. In this, an entire route through the station can be selected and all the associated points and signals along the route can be set at once by a switch for receiving, holding, blocking, or dispatching trains.

As an example, Old Delhi station has an RRI system from Siemens which allows selection from among 1122 possible routes. CR has a large RRI system at Kurla which controls signals from Ghatkopar to Sion on the Main Line, at Lokmanya Tilak Terminus, and from Chembur to GTB Nagar on the Harbour Line. The first route-relay interlocking system was set up on WR at Churchgate station control tower in the 1950s (equipment from Siemens?).

Currently [10/04], the numbers of stations with some form of electronic interlocking systems are: 18 on CR, 195 on ER, 263 on NR, 22 on NER, 43 on NFR, 55 on SR, 101 on SCR, 132 on SER, 44 on WR, 130 on ECR, 99 on ECoR, 154 on NCR, 13 on NWR, 71 on SECR, 34 on SWR, and 76 on WCR.

Regardless of whether the mechanisms are controlled manually or by electronic circuits, and whether they are operated mechanically or electrically, all interlocking schemes usually enforce several or all of the following rules:

No signal can be pulled off unless corresponding points are set correctly.
Facing points are locked to the corresponding route when a signal is pulled off.
Signals for conflicting movements cannot be pulled off simultaneously.
Points for conflicting routes cannot be set simultaneously.
Trailing points are locked to the rear when a signal is pulled off.
Distants, warners, repeaters, etc. cannot be pulled off unless the corresponding stop signals are pulled off.
Gate stop signals cannot be pulled off unless level-crossing gates are blocked to road traffic.
The description of the possible routes that can be set, and the corresponding dispositions of points and signals are found in the locking table and selection table for a station. The locking table lists the signals and points controlled; the levers at signal boxes (or control panels at control centres) which operate various signals and points; which signals and points are locked (and in what position) when other signals are pulled off or points set; which track circuits are clear or occupied; etc.

The selection table lists the allowed non-conflicting routes that can be set. The terms route selection, route locking, route holding, and route release are used to describe the various steps in the process of picking a route for a train.

In various semi-automated systems of interlocking the electrical or electromechanical mechanisms or the electronic circuitry takes over a large part of the bookkeeping details that determine the sequences in which signals must be pulled off or points set to assign a route to a train. In the more primitive mechanical interlocking systems, such a sequence has to be manually followed; for this purpose the locking and selection tables are used by the signalman, along with lever leads which indicate for each signal lever which other levers must be set or cleared.

RRI and PI equipment is from Siemens and some British manufacturers. In recent years interlocking accomplished by modern integrated electronic circuitry instead of electromechanical relay systtems has come into use (Solid State Interlocking (‘SSI’). [1/01] SSI is in place at 14 stations in India. SSI equipment is manufactured by RDSO. 210 stations have RRI installations, and 1970 have Panel Interlocking.

Source – IFRCA.org

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