The locomotives model doesn't jitter or anything, it just sometimes drives backwards when setting the lever to "forward".
I have no sure fire way to replicate it, yet, I'll update this if I find one.

Tends to depend on user position (when placed), how often you jiggled the speed lever, and just generally the way of the wind.

I mean on the bright side the model's direction seems to be stable, so this probably is a fixable issue :]

Try it with any of the current linking mods and you'd change your tune :D, but yes.