Roy wrote: I would expect the trucks under the adjoining cars to also have traction motors. No diesel prime mover, or electric generator. Is this correct?
No traction motors on the adjoining cars. The little yellow train is a Stadler GTW 2/6 (2 of six axles powered.)

Yes, I know, the Little Yellow Train has to climb 6% grades. I don't know how a 2/6 set up can do that either.
Sounds from the unit are pure electric plus view from the top shows no mufflers.

Little Yellow Train roof has no mufflers.

Videos:
https://youtu.be/EcmCu3DoHWA Sounds electric to me.
https://youtu.be/KgGLsDvAbCM Top view

