The UK has the British Transport Police. They are a branch of the police force dedicated to policing the railways. You won't normally see them on trains, but

The UK has the British Transport Police. They are a branch of the police force dedicated to policing the railways. You won't normally see them on trains, but they have offices in most big stations and if there's trouble then it'll get sorted out on the next stop. Normally you only see the police visibly present when there's a football match or other big event on, and the regular police are also there to deal with the crowds. Seeing trouble is a rarity though. Nowadays most stations have ticket barriers so it reduces the type of person who might dodge the fare. And most people know how to behave.