Ram Niranjan Singh was on his way to Pratapgarh to attend a family marriage when the incident took place in the Haridwar-Allahabad express train last night, they said, adding that his family members were also injured in the incident.
Singh, who was posted at Kiratpur in Bijnor district, had entered into an argument with some co-passengers after his suitcase hit a woman while loading luggage in Haridwar.
The issue got pacified for the moment with police intervention but when the train reached Bareilly, the rival group picked up the fight again and tried to drag Singh towards the gate of the coach.
The post-mortem has revealed the assailants shot Singh, Medical Superintendent of district hospital Vijay Yadav said.
Meanwhile, a manhunt has been launched to nab the absconding culprits, police said.