A gang of robbers who cut a hole in the roof of a train travelling through southern India have stolen almost $1 million in cash, police said.

The daring heist occurred somewhere along the more-than-300-kilometre-long journey, but authorities only realised the loot was missing hours after the express train arrived in the city of Chennai.

The train's mail carriage was carrying 3.4 billion rupees ($65.9 million) belonging to India's central bank and was being escorted by armed police seated in the next compartment.

Police suspect about six to eight robbers made a 1.2-metre-square hole in the carriage roof using a cutting torch before escaping undetected with some of the cash.

"We are inquiring with the train guard and the police personnel if they heard any unusual noises on the roof," police superintendent P Vijayakumar said.

Police said the robbers may have jumped off the train roof and that they were combing the tracks along the route in search of any of the missing notes.

"They created a manhole and then they sneaked one after the other and lifted the bundles to the roof before taking them away," the head of the investigation told AFP.

Another police officer was quoted in local media as saying the gang might have hidden themselves in the carriage before it was locked for the journey, before escaping through the roof.

Reserve Bank of India officials discovered the theft on Tuesday morning when they came to collect the cash from the carriage for disposal.

The remaining cash, although still usable, was soiled and was transported from the city of Salem to Chennai for destruction.

The audacious crime has echoes of the Great Train Robbery of 1963, when a gang of criminals in Britain stopped a night mail train and made off with 2.6 million pounds.

The train was carrying $65.9 million and was escorted by armed police. ( AFP: Arun Sankar )

AFP