A fire swept through a train car packed with sleeping passengers in southern India on Monday, killing at least 47 people and sending panicked survivors rushing for the only clear exit once the train stopped, officials said.

Investigators found charred remains of victims still in their sleeping berths and were struggling to identify them.

A railway station worker noticed the burning coach as the overnight train from New Delhi to the southeastern city of Chennai passed through the town of Nellore at about 4 a.m., local official B. Sridhar said. Nellore is about 500 kilometres south of Hyderabad, the capital of Andhra Pradesh state.

Once the alarm was raised, the train was stopped and the coach was detached from the rest of the train to prevent the blaze from spreading.

Passengers were evacuated once the train was halted.

"Since the fire had engulfed one door of the coach, people had to rush to the other end of the coach to exit," Sridhar said by telephone from the accident site.

Passengers said the fire spread swiftly through the coach.

"We woke up suddenly when the train came to a halt. That's when we noticed the fire at one end of the coach and shouted out. People were still sleeping, but once they realized the danger they scrambled to the door," Shantanu, a passenger in the coach, told New Delhi Television.

He said it took passengers a few minutes to react and push their way through the narrow space between the berths.

"By the time we got out of the door, the coach was completely on fire," said Shantanu, who like many Indians uses only one name. He said firefighters sprayed water over the burning coach, but it was gutted.

Sridhar said the fire may have been caused by an electrical short circuit in the coach.

28 hospitalized

The blaze killed 47 people, said Anil Kumar, a regional railway manager. At least 28 others were hospitalized with burns, including at least two in critical condition, he said.

Railway and medical workers were trying to identify the dead, he said.

"This is a very difficult task, since some of the bodies are charred beyond recognition," Sridhar said, adding that officials were making preliminary identifications based on the reservations chart from the train's records.

Prime Minister Manmohan Singh conveyed his condolences to the families of the victims.

India has one of the world's largest train networks, with more than 10,000 trains and 64,000 kilometres of track. Around 20 million people travel by train each day.