Initially, the signalling was done manually using flags during day and oil lamps during night.

A railway interlocking system controls the traffic in a railway station and between the adjacent stations. The train movement is allowed in accordance with the rules and regulations governing the movement of trains.

The request to set a route or operate a signal or a point comes from the operator, who is a signalman, but the decision to allow the move is made by the interlocking system on the basis of the existing field conditions and the inbuilt safety logic. The final goal is to ensure safe passage of train through the controlled area.

The first interlocking systems were purely mechanical which then evolved into several power-interlocking systems. The next important milestone, the 'Relay Interlocking' was developed in 1927. A medium sized interlocking consists of 2000-3000 relays while a large signalling scheme may require 30,000 relays or more. The relays used for vital signaling application are expensive to manufacture, test and calibrate. They need large and expensive enclosures and a lot of cumbersome wiring. Any modification involves changes in the wiring and this may cause hold up of traffic for long intervals.

Over the past few decades, the development in microprocessor technology has made rapid strides and processor-based equipment is finding application in every kind of modern transport systems. Computer technology is characterised by rapid innovation cycles, increasing performance and a better cost to performance ratio. With the introduction of Solid State Interlocking (SSI) system in 1985, the safety critical railway signaling systems could take the advantage of the ever-increasing power of microprocessors. British Railways Research developed the SSI in joint venture with UK railway supply industry. The name ' Solid State Interlocking' system was used for British Railways Electronic Interlocking system, but has become generic for Electronic Interlocking Systems like 'Xerox' and 'Hoover'.

The Electronic Interlocking system (EIS) offers a lot of advantages over the conventional relay based interlocking which is widely used in Indian railways at present. An EIS occupies less space, consumes less power, is easy to install and maintain and is cost-effective. The interlocking logic in the EIS is based on software and hence any modification is easy without the need for any wiring changes. This eliminates the need to block traffic for long intervals whenever there is need for system up-gradation or modification. The EIS are processor based systems which have extensive diagnostic tests built into them. This improves the reliability of the system and leads to minimal system-down-time even in case of failures.

The faulty module can be located easily and replaced with a spare one. Being a safety critical system where the human lives are at risk if the system fails, the EIS is required to achieve high levels of safety that was possible with relay logic or do even better. Keeping in view the continuous flow of train traffic, the EIS has to be functional continuously and has to meet high availability standards. The probability of failure has to be less than one in ten million (10-7) and the system has to be fail-safe. A fail-safe system does not lead to any safety hazards even if it fails because all the outputs are forced to safe state. Fail- safe in railway parlance means the system shall put the signals to danger and will not move any switch in case of any failure. With the advancement in fault tolerant computing and fail-safety techniques, the processor based Electronic Interlocking Systems have been successful in achieving the high levels of reliability, availability and safety required for safety critical signaling systems.

In view of the numerous advantages the Electronic Interlocking System offers over traditional relay based implementation, the railways in advanced countries of Europe, North America, Japan and Australia have gone in for large-scale introduction of Electronic Interlocking Systems. Some of the manufacturers of Electronic Interlocking Systems whose products are being used worldwide are Union Switch and Signals, Bombardier Transportation, Siemens Transportation Systems, General Electric Transportation Systems, ALSTOM, Westinghouse Rail systems, Kyosan and AZD Praha.

Medha Servo Drives is entering the field with the development of its Electronic Interlocking System MEI633. MEI633 is a multiprocessor system with 100 per cent hot standby for all the vital modules. It is a modular, distributed, fail-safe system that has been designed to meet CENELEC Safety Integrity Level 4 requirements. (CENELEC is the European Committee for Electro-technical Standardization defining the requirements for acceptance and approval of safety related electronic systems in railway signalling field. Safety Integrity Level 4 is the highest level of safety as defined by CENELEC).

Medha's EIS is a highly configurable system which can be used for small wayside stations as well as large stations. It can be configured in accordance with the size of the station being controlled, the layout of the station yard and the rules and regulations of the individual railways. The system software consists of two parts - a generic component which remains same for all the systems and a configurable component which can be modified as per the application requirement. The configurable component can be programmed using a highly user-friendly Compiler.

With enormous built-in intelligence, and capacity to hold large amounts of information, processor based electronic systems can lead to high levels of automation in signaling. The introduction of Electronic Interlocking System is a major step in this direction, which avoids human errors in signalling and consequent accidents.

In advanced countries, the EIS has already found widespread application. Slowly, but surely, developing countries also will be able to take the full advantage of technology in signalling applications with the use of cost-effective Electronic Interlocking Systems.

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