Originally Posted by Aviatrix

Getting on a train without paying is indeed a foolish thing to do (as well as being dishonest, of course - it's theft, purely and simply).

You may be able to get away with it... but if you get caught you'll be in trouble.

Unless things have changed since I last read up on this the rules for getting on a train without a ticket in Germany are as follows:

If the ticket office at your station was closed you can buy a ticket on the train and there is no surcharge - but you MUST report to the train attendant as soon as practicable (i.e., don't sit down and wait for him/her to find you, but make an active effort to go and look for him)

If you get on a train without a ticket for other reasons (for example because you didn't have time to buy a ticket) then you can buy the ticket on the train for a small surcharge - but again you MUST report to the train attendant as soon as practicable, see above

If the attendent catches you without a ticket you are deemed to be a fare dodger and you get fined.

So - don't risk it, get yourself a ticket. Personally I would buy a ticket online - full fare tickets are valid on any train, so it doesn't matter if your flight is late and you end up going on a different train from the one you had planned to catch.