Dendrophagus Colossus

The Giant Tree-eating Snails do exactly that – consume large quantities of wood. They are most often encountered roaming the outskirts of towering forests, so as to have enough food, yet not to venture too deep, where trees get too dense for their enormous shell to pass.

No one, of course, dares to attack such a giant, so the first settlers have found it to be a perfectly safe house-on-tracks.