The duo were trying to cross over to the other side of the railway track when they were hit by the weekly New Jalpaigudi-Darbhanga train, Ballia police station's Inspector Harendra Prasad Singh said.
While an SSB jawan, identified as Devendra Singh (25) was killed on the spot, his colleague Kuldeep Singh (25) suffered injuries, he said.
The body has been sent for post-mortem, while the injured got admitted at a hospital in Begusarai.
The deceased hailed from Jhansi district of Uttar Pradesh, while the injured SSB jawan was a native of Jyotiba Phule Nagar district of UP, Singh said.
The SSB jawans were posted at the central paramilitary force's 27th battalion at Birpur on Indo-Nepal border in Supaul district.