The Mysuru-Darbhanga Bagmati Express collided with a freight train on October 11.
The joint inspection note of a passenger-freight train collision near Chennai last week, prepared by seven senior railway officials, suggests the train may have derailed at the intersection point.
Another report by a three-member committee of senior railway officials had earlier raised the apprehension of sabotage after finding some loosened or opened parts during a site inspection.
However, the contents of the joint note, prepared soon after the accident, do not mention any sabotage angle.
A railway spokesperson said the joint report cannot conclusively state the reasons for the accident but it can be one of the inputs for the Commissioner of Railway Safety to prepare the final investigation report.
The Mysuru-Darbhanga Bagmati Express rammed into a stationary freight train at the Kavaraipettai railway station in the Chennai rail division at around 8.30 pm on October 11, leaving several passengers injured.
"The experts who have prepared the joint note didn't say that they found any mechanical part opened or loosened at the accident site. Instead, they found nuts, bolts, rails, tongue rail, and other similar items in broken condition," said a safety expert after analysing the inspection report.
He added, "The kind of damage these seven officials have recorded in the joint note goes on to show that the train derailed at the intersection point of the main line and the loop line." The safety expert's assertion gets support from the data logger's yard simulation video in which it appeared that the train was moving on the main line as well as the loop line.
"As the train can move in only one direction, the data logger's yard simulation video hints that it might have derailed at the interlocking point. While the engine and some coaches headed towards the loop line and collided with the freight train, the remaining coaches scattered all around, infringing the main line as well," KP Arya, who retired as the chief signal and telecom engineer/information technology in the Northern Railway, said.
Mr Arya, who looked into the joint note, raised the apprehension of engineering defect of tracks and interlocking-point mechanisms for the derailment.
He said when interlocking is done for the trains to change track, a small gap (but beyond the acceptable limits) remains at many points between the stock rail and the middle or later part of tongue rail due to which stones, small wooden blocks, and other similar items get stuck.
This issue at times leads to derailment of trains as the monitoring system fails to track the entangled objects between the stock rail and tongue rail, according to Mr Arya.
"The joint note said that they found a crushed gunny bag and a piece of tongue rail from the accident site. There are chances that the gunny bag might have come between that gap of tongue rail and the stock rail," he added.
Initially, the railway officials, while talking about the possible cause of collision, had held that the passenger train was given a green signal for the main line but it entered a loop line and hit a freight train already waiting on the loop line.
Safety experts had blamed the lack of coordination between the signal system and interlocking to some fault in the signalling system.
This is unusual as, according to safety experts, in an interlocking-signalling system, the signal aspect follows the interlocking of tracks which means if the signal is green for the main line, the interlocking will automatically be set in such a way that the train comes on the main line.
A senior railway official said, "The data logger's video, the joint note and the crew's statement that they experienced a jerk at the interlocking point, all tend to show that the train derailed at the intersection point. So did this derailment happen due to mechanical error or some act of sabotage? I think it will come out only after the ongoing investigation by the Commissioner of Railway Safety as well as National Investigation Agency."