I passed my Cat. G test a few years ago on a Fowler Road Roller. Owned by a museum I worked at. Spent weeks trundling around town with L plates on practising 28 point turns, hill starts AND emergency stops hahahahahahaha! (Throw it in reverse!)
Test was great. Testers walk alongside and tell you to do the various things. I was told by my instructor that it helps to have arms like an octopus. You are on the footplate alone. You have to maintain a decent fire, work the injectors to keep the water in the boiler up to the mark, open/shut the regulator, select the correct gear BEFORE you move off. Oh yes and steer it as well. Don't forget the hand signals too. Watch the pressure gauge like a hawk also.
The fright came at the very end. Questions on the Highway Code !! It's many, many years since I passed my car test. By pure chance I knew the answers. Stopping distance at 30mph - DREAM ON Baby
Yes, Cat.G covers steam or diesel. GO FOR IT !! It's a laugh! If you can borrow a steam roller you'll be black as night when you finish your test - BUT you'll have a grin on your face a mile wide !! Fred Dibnah Mk.2. Did you liiike that ??
Sorry. Long winded post.