The incident took place a small distance away from the Kiul railway station in Jamui, a sensitive route which has already witnessed three major incidents of train robbery in the last week itself.
Passengers alleged that the guards on duty inside the train chose to hide instead of apprehending the robbers.
The Railway Police, which arrived nearly 30 minutes later, fired 12 rounds at the robbers who managed to escape.
On Friday, a jawan of the Bihar Police was shot dead and another was another critically injured on board an express train that was passing through the state's Munger district, as they tried to apprehend suspected robbers.
On December 29, a gang of armed robbers looted passengers of valuables worth lakhs of rupees inside the Jharkhand-bound Patliputra Express after stopping it along the Bihar-Jharkhand border.