Explained | What is the electronic interlocking system in railways? 

Understanding how the electronic interlocking system works and what led to the tragic train accident in Odisha’s Balasore.

June 05, 2023 08:09 pm | Updated June 08, 2023 10:58 am IST

More than 250 people were killed and over 1,000 injured in a multi-train collision near the Bahanaga Bazar station in Balasore, Odisha on June 2.

More than 250 people were killed and over 1,000 injured in a multi-train collision near the Bahanaga Bazar station in Balasore, Odisha on June 2. | Photo Credit: PTI

The story so far: An electronic track management system used by the railways has become the focus of investigations after last week’s horrific train crash involving two express trains and a goods train in Odisha’s Balasore district that left 275 passengers dead and more than 1,000 injured

Earlier this week, Minister for Railways Ashwini Vaishnaw told the media that a “change made in the electronic interlocking and point machine” had led to the accident. “The Commissioner of Railway Safety has investigated the matter… we have identified the cause of the incident and people responsible for it. It happened due to a change in electronic interlocking,” the Minister said, while insisting that the crash isn’t linked to the Kavach anti-collision system.

The Railway Board, the apex body of the Indian Railways, also singled out “signalling interference” in its preliminary probe, with senior officials indicating that possible sabotage and tampering with the interlocking system could have caused the mishap.

What is an interlocking system?

Railway traffic is controlled and managed by railway signalling. Interlocking, an integral part of railway signalling, involves a set of apparatus placed on a track to manage the movement of trains and track configuration at stations and junctions. It prevents conflicting movements as a train gets a signal to proceed ahead only when its route is set, locked, and detected as safe. 

The signal apparatus in an interlocking system may be interconnected mechanically or electrically with the tracks or both. Electronic interlocking (EI) is an advanced version of signalling that uses computer-based systems and electronic components to control signals and points. The Indian Railways defines it as a “microprocessor-based interlocking equipment to read the yard and panel inputs; process them in a fail-safe manner and generate required output.” Unlike the conventional relay interlocking system, the “interlocking logic” in an EI system is managed via software and electronic components. 

As of last year, 2,888 stations in India were equipped with an electronic interlocking system — comprising 45.5% of the Indian Railways network.

Three components that form the basis of an electronic interlocking signal system are:

Signal: Based on the status of the track ahead, light signals direct a train to stop (red light), proceed (green), or exercise caution (yellow).

Point: These are movable sections of a track which allow a train to change track by guiding the wheels towards a straight or diverging line. They are operated using switches. For instance, if a train has to change lines, the point is activated ahead of time and locked. A point machine is used to lock point switches.

Track circuit: These are electrical circuits on tracks installed to detect the presence of a train. These verify whether it is safe for a train to proceed over it.

How does the system work? 

Two information points form the basis of the EI system. A signal to pass is given based first on which direction the track is set, and second on whether the divergent track is free of obstruction. 

Once a route is determined, it is accordingly aligned, and points are locked at a particular position. The train gets a signal indicating whether it should move straight or switch to a new track. The EI system then directs it to the empty track at the point where two lines meet if it is required to switch. In the meantime, circuits prevent another train from running on that block. All points remain locked until the train has crossed a particular section of the track in use or the signal to proceed has been withdrawn. In case there is a problem in the system, a red light is flashed, indicating that the route ahead is not clear or safe. 

Like the black box of aircraft, all activities in the signalling system are recorded in a system called a ‘data logger’.

What led to the Odisha crash?

A signalling failure is suspected to have caused the accident. The Coromandel Express heading to Chennai from Kolkata was initially given the green signal to enter the Up Main Line, but the signal was then taken off, as The Hindu reported earlier. The train did not have a scheduled stoppage at the Bahanaga Bazar station, which it was approaching when it moved out of the main track and entered the loop line, which is a side track used to accommodate goods trains. It crashed into a parked freight train carrying iron ore, triggering the multi-train collision.

At the time, the Coromandel Express was running at a speed of 128 kmph (80 mph). The impact of the crash was such that the engine of the Coromandel Express and the first few coaches jumped the tracks, toppled and hit the last two coaches of the Yeshwantpur-Howrah train heading in the opposite direction on the Down Main Line, running at a speed of 126 kmph.

It is, however, not yet clear why the signal was given and taken off, and whether the signal was showing ‘green’ or ‘red’ when the Coromandel Express crossed it.

As to why the EI system did not prevent the Coromandel Express from moving to the loop track, senior railway officials claim that the “error-proof” and “fail-safe” system could have been tampered with.

“It is called a ‘fail safe’ system, so it means that even if it fails, all the signals will turn red and all train operations will stop. Now, as the minister said there was a problem with the signalling system. It could be that someone has done some digging without seeing the cables. Running of any machine is prone to failures,” Jaya Verma Sinha, Member of Operation and Business Development, Railway Board, said in a briefing on the crash on Sunday. “99.9% there is no possibility of the machine failing but there is a 0.1% chance of failure,” she said, and added, “That possibility is always there in all kinds of systems.” 

The driver of the Coromandel Express, meanwhile, has been given a clean chit. Officials said the driver was within the speed limit and had not jumped any signal.

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