The ghastly incident took place in Lakhisarai district Tuesday.
Virendra Kumar Tyagi was first thrashed by dozens of passengers at the Patna railway station but he survived, Government Railway Police inspector Rajendra Paswan said.
As he boarded the Danapur-Bhagalpur Inter-City Express, some passengers again attacked him, killing Tyagi. The officer was not on duty and was not in uniform either.
J. Jayalalithaa passes away: Her life in pictures
Photos: Shah Rukh Khan, Deepika Padukone, Alia Bhatt at an awards show
Photos: Big B, Iulia Vantur at Himesh Reshammiya's album launch
Photos: Aamir Khan with 'Dangal' actresses at fashion show
Spotted: Twinkle Khanna, Shriya Saran at an event in Mumbai