Shamus,sorry to hear of this woe.
Shamus, I've had a similar problem some years ago with a Bachmann Niagara, and a Mantua (junk brand) Camelback.
The solution for the Bachmann was to take the entire thing apart and reassemble it. The fix-it man at the club at that time said this was his SOP. From his trouble-shooting experience, he claimed that about seven out of ten times, you will not find anything out of order, yet the mechanism will function without any trouble once reassembled.
The Niagara was mysteriously cured this way. We tried that with the Camelback and it failed, and I must send it out for repair. The Camelback has a worm drive, and the motor doesn't seem to line up correctly with the drive gear.
Let's hear what the manufacturer says. Do you have a competent Bachmann authorized service centre available nearby?
George.