India's state-run rail system, the world's fourth-largest and a lifeline for millions of poorer travellers, has struggled under growing demand and a lack of investment in upgrades.

The train, travelling from the eastern city of Patna to the northeastern city of Guwahati, left the tracks in the district of Alipurduar late on Tuesday, the spokesman said.

"A full investigation will take place," Anil Saxena told Reuters.

India's deadliest rail crash in years on November 20 prompted a request from the railways to the finance ministry for about $17.5 billion to set up a dedicated safety fund, in addition to record investment pledged by Prime Minister Narendra Modi's government to modernise the network.