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 electro-pneumatic systems, and computerized circuits in the newer electronic systems.
(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. When the signal/switch button and the route button are operated simultaneously, concerned route initiation relay, LR picks up. Every signal will have as many LRs as that number of routes and alternate overlap and routes.
The RRI’s main operating component is the control panel which is provided with switches and buttons for operation of all functions as signals, point siding etc. Geographical demarcation of track circuits, points, signals and all other functions with illuminations as per their respective position is provided. A counter for registering emergency operation is provided. Indicators for power supply availability, with acknowledgment buttons, are provided.
Conditions like the train on-line and train arrival are also available. One main feature of route relay interlocking is that the buttons by which routes are defined are placed directly on the track diagram, rather than underneath the diagram as on CTC panels or conventional interlocks. Route relay panels generally display more dynamic information about the state of an interlocking than the track occupancy lights associated with the earlier system.
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).
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.
- Distant, 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 centers) 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. 247 stations now have RRI installations and the number of stations with Panel Interlocking.
EI (Electronic Interlocking) :
Modern interlocking (those installed since the late 1980s) are the generally solid state, where the wired networks of relays are replaced by software logic running on special-purpose control hardware. The fact that the logic is implemented by software rather than hard-wired circuitry greatly facilitates the ability to make modifications when needed by reprogramming rather than rewiring. In many implementations, this vital logic is stored as firmware or in ROM that cannot be easily altered to both resist unsafe modification and meet regulatory safety testing requirements.
At this time there were also changes in the systems that controlled interlocking. Whereas before technologies such as NX and Automatic Route Setting required racks and racks of relays and other devices, solid-state software-based systems could handle such functions with less cost and physical footprint. Initially, processor-driven Unit Lever and NX panels could be set up to command field equipment of either electronic or relay type, however as display technology improved, these hard-wired physical devices could be updated with visual display units, which allowed changes in field equipment be represented to the signaler without any hardware modifications.
Solid State Interlocking:
(SSI) is the brand name of the first generation microprocessor-based interlocking developed in the 1980s by British Rail, GEC-General Signal and Westinghouse Signals Ltd in the UK. Second generation processor-based interlocking are known by the term “Computer-Based Interlocking” (CBI), of which MicroLok (trademark of Union Switch & Signal, now Ansaldo STS), Westlock and West race (trademarks of Invensys Rail) and Smartlock (trademark of Alstom) are examples.