This one shows it on a railway wagon and makes it clear that it has treads - that it's a tunnelling machine rather than a really weird submarine. This isn't going to be known at the start of the adventure, of course, and the adventurers will have to work pretty hard to learn this much.

This looks a little phoney, and I may mess with it a little more. Also needs some troops guarding it. Comments please...
Update 17/8/04: Now messed with, troops added, etc.