What led to Mysuru-Darbhanga Express-freight train collision in Tamil Nadu?

Tamil Nadu Train Accident: At least 12 coaches of a Bagmati Express train which runs from Mysuru to Darbhanga derailed after it rammed into a stationary train at Kavarapettai in Tamil Nadu on Friday.
The accident took place after the express with LHB coaches crossed Ponneri station at 8.27pm near Chennai in Tiruvallur district.
Rescue teams and ambulances have rushed to the spot.
According to a preliminary inquiry, a switch to the loop line, instead of getting into the main line, could have resulted in the collision.
The train was running at 75 kmph speed when it hit a stationary goods train. When the driver began slowing down, the train entered in the loop line and collided with a goods train. 
The crew also experienced ‘heavy jerk’ after the train entered the loop line.
“While entering the Kavaraipettai station, the train crew experienced heavy jerk and instead of going into the main line as per the signal given, the train entered into the Loop/line@75kmp & hit the Goods train standing in loop line,” reported PTI quoting Southern Railway press release.
“The pilot and loco pilot are hale and hearty. The injured were immediately moved to the nearby hospital,” said RN Singh, General Manager of Southern Railway.
Meanwhile, Dilip Kumar, Executive Director of Information & Publicity (ED/IP), Railway Board, said that all passengers have been evacuated from the entire train. 
“No casualties have been reported and no one is reported to be seriously injured. Special arrangements are being made by the railways to take all the passengers to their destination. During the entire journey, the security and food arrangements of the passengers will be made by the railways,” Kumar added.
The fire department is engaged in the work of removing the accident train coaches.
“I was shocked to know that there was a train accident in Kavaripettai, Tiruvallur district. As soon as the information was available, I ordered Minister SM Nasar and other government officials including the District Collector to go to the accident site. The government is working fast in rescue and relief work. The injured are immediately taken to the hospital,” said CM MK Stalin in a post on X.
Stating that he is monitoring the rescue operations, the Chief Minister further said, “A separate team is functioning to arrange food and travel facilities for the other passengers to return home.”
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— Medical and railway assistance teams are at the site to provide help to passengers.
— The divisional railway manager of Chennai and the general manager of the Southern Railway Zone have left for the accident site.
— The railways is also making arrangements to send people to their respective destinations by making alternate arrangements
Passengers are being moved to Chennai Central by EMU, and a fresh train has been readied at Chennai to take them further to Darbhanga/other destinations.