On the evening of June 2, the Coromandel Express passenger train was moving at 128 kph (80 mph), heading for the southern city of Chennai, when it approached Bahanaga Bazar station in the eastern Indian state of Odisha.

What followed was India’s deadliest rail crash in more than two decades as the train careened into a stationary goods train resulting in a three train collision and derailment killing 288 people and injuring more than a thousand.

Reuters analysed drone footage, carriage layouts and schematics to show how the cheapest and most crowded carriages bore the brunt of the impact of the crash in the Balasore district of Odisha. An Indian Railways spokesman confirmed that the cheapest carriages were among the worst affected by the disaster.

In their first detailed briefing on the crash on June 4, Indian Railways officials said investigators were focusing on the failure of the track management system.

The computer-controlled track management system, called the “interlocking system”, normally directs trains to an empty track at the point where two tracks meet, Sandeep Mathur, principal executive director for signalling for the Indian Railways, told reporters.

On June 11, Reuters reported that an official probe by the Commission of Railway Safety (CRS) is focusing on suspected manual bypassing of an automated signalling system that guides train movement, three Indian Railways sources told Reuters.

CRS investigators suspect the bypass was done by railway workers to get around signalling hurdles that arose from a malfunctioning barrier used to stop road traffic at a nearby rail-road intersection, two of the three sources said.

The CRS, which is India's rail safety authority, did not respond to a request for comment.

The carriages worst damaged on the two passenger trains were the non-air conditioned ones, according to Reuters’ analysis, some of which were unreserved coaches that allow anyone holding the cheapest category of ticket to board without a specific seat reserved for them. Such carriages are often the most crowded.

These unreserved and general coaches are often used by migrant workers crossing India. Train travel is a lifeline for many of them. Long journeys on trains like the Howrah and Coromandel Express are vital for them to reach prosperous parts of the country from their remote villages.

Using drone footage of the aftermath of the crash and Indian Railways carriage data, Reuters found that the Coromandel Express typically has seven non-air conditioned carriages, including two unreserved carriages, while the Howrah Superfast Express has nine non-air conditioned carriages, two unreserved. Another unreserved carriage on both trains is partly used as a luggage carrier.

An Indian Railways spokesperson said the engine, parcel van and two general coaches of the Coromandel Express were the worst affected in the crash.

On the Coromandel Express, the general class carriages were closest to the engine and were strewn across the tracks after the collision with the stationary goods train. On the Howrah they were the farthest from the engine, and were hit by the derailed Coromandel Express carriages.

Unreserved and general class are the cheapest forms of railway passenger transport in India and seats are often scarce due to overcrowding.

Safety on India’s trains has improved steadily over the past two decades according to the Indian Railways Yearbook, an annual report that lists accidents and casualties.

India's deadliest railway accident was in 1981, when a train plunged off a bridge into a river in Bihar state, killing an estimated 800 people.

The administration of Prime Minister Narendra Modi, who took office in 2014, has launched high-speed trains as part of its plans to modernise the network. Some critics have said it has not focussed enough on safety and upgrading ageing infrastructure.

The Railway Ministry spokesperson declined to comment on the safety of its trains.

Connectivity to the line was restored within three days, but satellite imagery from June 6 and drone footage June 4 show multiple carriages on both sides of the railway line.

The site of the accident is rural. Few trains stop at Bahanaga Bazar, the sleepy, small rural station in Odisha’s Balasore district.