Could my Lionel TPC 300 have scrambled my MTH Engine's brain ?

Did you follow the instructions in the TPC unit manual to set it for conventional control?

There are also specific keystrokes to access features of PS,  PS1 and PS2 locomotives. You need to know which version of PS you are operating.

Rick,  Yes,  I followed the instructions for the TPC,  It worked great with Lionel Post War engines.

I don't have any idea what version of PS the engine is,  no box or manual !

Since the TPC runs Lionel conventional engines OK, I would assign fault with the electronics in the Yellow Belly.  MTH produced that engine in PS-1, PS-2, and PS-3.  Brain-dead PS-1 electronics typically start sounds but won't move the engine out of neutral.  Can't speak to PS-2 and PS-3 board behavior.

The TPC's output is chopped wave A/C.  Same as the MTH Z1000 and Z500 transformers.   Have you tried running the loco with sine wave power direct from the MRC transformer?

https://mthtrains.com/30-1412-1

The only one with that cab number

Thanks for the item number Joe.

Tracker John,  yes, that is how I'm getting the strange behavior, straight from the transformer.

If you click on the MTH link provided above, you can find the user's manual. Then look up "reset to factory default".