AIUI, it must travel by bicycle (though I suspect we'll extend that definition to any form of human powered vehicle[1]), with the exception of water crossings. So ferries are allowed, trains very much aren't.
[1] There was some debate as to the validity of SmallestCub's pushchair short-wheelbase, fully faired, direct-drive, recumbent tandem quadricycle for the purpose. We reckoned it was broadly in the spirit of the thing.