There is usually a machine that inspects the ticket on entering the platform. There is usually a machine that inspects the ticket on exiting the platform, and there is sometimes a person walking along the train asking to see everyone's tickets. The machines are backed up by people, if there's no-one manning the line of barriers then they're just left open, and they act as a fallback for damaged tickets and the like, as well as someone you can plead your case to if you've lost it. Generally if you try to enter without a ticket you'll be turned away. Trying to leave without a ticket will be harder without paying a fine, as will getting caught without one on the actual train (again, you can try to plead your case, and there are legimitate reasons like the ticket machine at the station you left from not working, in which case you can buy the ticket at your destination, but you'll need to be believed).
