About eight robbers, who boarded the Gorakhpur-Sonepur passenger train at Sugauli station, fired nine rounds on the passengers injuring three of them, one of whom died later in a hospital, official sources said.
The robbers looted cash and belongings from some passengers before escaping, they said.
The deceased has been identified as 35-year-old Chandrabhushan Prasad, they said, adding the injured are his brothers Damodar and Haridarshan Chaudhary.
Four members of the escort party Hawildar Arundev Rai and constables Dharamveer, Munna Pandit, Oril Mahto have been suspended on charges of dereliction of duty, Superintendent of Police (Railway) Sheo Kumar Jha said.
ADG (Railway) P N Rai, IG (Railway) Vinay Kumar and other senior railway and police officials visited the spot and met the injured.
An FIR has been lodged and three teams have been set up to raid the surrounding areas, sources said.
