The body of Dharamvir Singh (45) was noticed by other passengers in the compartment when the train reached Bijni, police said.
The passengers informed the station authorities at Rangiya.
Singh, a native of Sonepat in Haryana, was posted in Manipur.
Police said he was returning from home and had boarded the Delhi-Dibrugarh Brahmaputra Mail on November 28. Though the cause of his death is yet to be ascertained, authorities suspect some ailment.
Rs 2,400 in cash, PAN and identity cards, clothes and some medicines were recovered from his baggage, the police said.
Senior Assam Rifles officials have reached Rangiya.
