What made the police suspect George of being a criminal?

George didn't have his ticket for the train (Harris had it), so he jumped on the train after it had started to move, without a ticket, after he'd specifically told by an official not to do so. After that, he had the audacity to move up into a nicer section of the train than he'd paid for and refused to pay the difference. You can find the details for each of his mistakes (sins) on page 277.

Source(s): Three Men in a Boat (To Say Nothing of the Dog)