Both my MTH PS-2 Amtrak Genesis went this way. I was running at 79 mph per the DCS hand held controller, and as I crossed my Atlas O double tracked truss bridge, a loud bang was heard, like a fire cracker or a scale shot gun blast!. The locomotive that suffered the failure immediately lost all of its tires as the lead unit continued to pull. i immediately stopped the train as soon as I could. I also smelled a very strong electrical burning odor. I was running mine together in a lash up using DCS pulling a 7 car K-line 21 inch Amtrak set and had only completed two loops in a layout that measured 20 by 13 running on gargraves track with ross custom switches.
Once I opened the locomotive I found what appeared to be a capacitor cover loose inside. Apparently this is what caused the loud explosion inside the locomotive as the capacitor had a catastrophic failure. The plastic car body was warm to touch aft of the cab area as you have described on your unit. It may be possible that your unit is about to have a similar failure. Soon after this happened, my second MTH Amtrak genesis had the same failure as it was pulling 5 Atlas O Horizon cars. Now I have two dead PS-2 Genesis that I have to decide how to get repaired. I am a bit apprehensive about having more PS-2 boards installed after two consecutive failures. I also had a PS2 SW1200 fail this way also and surged like you described and the shell was abnormally warm. I love the K5LA horn on the MTH genesis locomotives, but I am leary about spending more money on PS2 since I have had so little luck with it. Maybe you can contact MTH customer service for help to see what they have to say.
