I had an odd one on the bench, something that I didn't realize was possible.
I have an MTH Imperial FEF for repair. Once I got everything working in command mode, I tested it in conventional mode. I cracked the throttle so it got 6-7 volts and it powered up. After it sat for 30 seconds or so, I cycled direction and I got the startup hissing chuffs and corresponding chuffing smoke. Then the standard chuff sound started, again with the chuffing smoke.
The wheels aren't turning! I kid you not, no wheels!
OK I think, the driveshaft has come loose and just the flywheel is rotating, off comes the shell.
I do the same test again, same result, only the flywheel isn't moving at all! I was pretty surprised, I had no idea the electronics would generate the sounds without the tach sending feedback! It was just like the locomotives was rolling down the tracks, chuff sounds, smoke chuffing away, and no motion!