Well having four separate power supplies would seem to rule a failing / over loaded power supply out as the root problem. That would bring us back to the LM or the motors.
After the loco's stopped how long did the sound continue play for? less than a minute? the whole 15 minutes stopped time? Just wondering.
I find it strange that if dirty wheels and track was the root problem that you could repeat the situation (run 10 minutes, slow, LV warning, stop, wait, run 10 minutes, slow, LV warning, stop, wait ad infinitum) without the run time before the slowing, LV warning and finally stopping getting progressively worse as the wheels etc dirtied up more and more each time.
- Tim