Assistant sub-inspector Mohar Singh was being held by the Government Railway Police of Haryana, a senior police officer said.
Singh has been arrested under sections 285, 336 and 337 of the IPC relating to accidental firing by an officer while on duty, the Government Railway Police officer said.
The jawan, who was on duty in the New Delhi-bound Shatabdi Express from Kalka when the incident occurred, was being questioned at Kurukshetra where the train was halted to rush Sandeep to a city hospital.
The hockey star was later shifted to PGI, Chandigarh, where he was operated. GRP, Haryana, will also take photographs of the railway seat and blood samples.
The RPF officer, who was on duty in the train, had fired the shot accidentally from his service pistol, the GRP officer said.