A train to London was evacuated after a passenger reportedly pulled a knife and threatened a conductor.
The train had just pulled into Haddenham and Thame station when the passenger was pinned to the ground by four police officers and arrested, a witness said.
Passengers were then told to get off the train and join another that was heading towards the capital.
A 23-year-old man was arrested at the scene on suspicion of ABH and is currently in custody, police said.
A witness told the Daily Mail: “Around 50 people had to get off the train and I saw four police officers sprinting across the bridge to get to the man.
“They got onto the carriage, which was next to the one I was on, and managed to pin him down.
“When we were waiting on the platform, a couple who were on the same carriage as the man said that he had a knife.
“They said he was shouting that he wanted to get into the driver’s carriage.”
The witness added that around four police cars and two riot vans were outside the station.
A spokesman for British Transport Police said: “We were called to Haddenham and Thame Parkway station shortly after 5pm following reports of an assault on a train guard.
“Our officers attended and a 23 year-old man was arrested at the scene on suspicion of ABH and is currently in custody awaiting interview.”