The rifleman alongwith two others began to molest the lady as soon as the Dibrugarh-Delhi Brahmaputra Mail chugged out of Ara station.
The woman ran for her life inside the B1 coach, with other commuters remaining mute spectators. She tried to hide in the washroom, but the molesters stopped her in the tracks, said Kamal, a coach attendant.
Finally, the woman jumped from the train and fell on the railway track. It was then that a commuter informed the TTE and RPF, and the woman was found on the rail track, with severe head injuries, unconscious.
She was rushed to Ara hospital, where she is currently battling for life.
Meanwhile, the three molesters including the J&K Rifles jawan Ramesh Kumar were nabbed at Buxar station by the GRP, said M R Naik, SP Bhojpur.
IANS adds: Patna Railway Superintendent of Police Suresh Choudhary said the victim was seriously injured when she jumped out of the Delhi-bound Brahamputra Mail between Bihta and Ara stations.
The critically injured woman is said to from Darjeeling in West Bengal and boarded the train at Guwahati in Assam, a police official said.
Choudhary said police were yet to record the victim's statement as she was in a serious condition.
The incident took place on a day the Bihar Police chief called for speedy trials of cases involving rape and eve-teasing.
Inspector General of Police Arvind Pandey said all district superintendents of police and railway superintendent of police had been told to expedite investigation and file chargesheets at the earliest.
"The state government has made it clear to police officials to deal seriously with rape and eve teasing cases," he told the media.
Till October 2012, 482 cases of rape and 183 cases of harassment of women were pending in Bihar.
Pande said the Bihar government was serious in setting up such courts, and soon.
