The deceased was shot by the robbers when he resisted the robbers on board the running train near Jhajha railway station, about 160 km from here, a police official said.
The victim was identified as Shivjee Gupta, a resident of Howrah in West Bengal, who was going to Lucknow for his son's examination.
Another passenger, who was seriously injured, was admitted to the railway hospital in Kiul for treatment.
The robbers looted cash and valuables at gun point from passengers before they got down after pulling the alarm chain.
Police officials in Jhajha said the robbers boarded the train at Jasidih railway station and began looting the passengers at gun point as soon as it moved out of the station.
A case was lodged at the government railway police station at Jhajha.

Train robberies have become common in Bihar. Armed robbers looted dozens of passengers on the Janseva express last month. In August, a passenger was shot dead by a gang of five armed men while a dozen handicapped passengers were looted four months ago on another train.