Once you're on the train, there is little the conductor can do.

He or she likely will be completely satisfied that the travel distance is covered by a ticket.

It is FAR too much work to call the police to have the passenger removed, etc.

I admit I don't quite understand the problem, but as long as the train stops to pick you up, a ticket covering a longer travel distance should be just fine. All you say, if asked, is that you changed your mind as to the boarding station. The crew is unlikely to be well-informed about all the tariff idiosyncracies and cares about one basic thing: does the passenger have a ticket covering the distance they're going.