I was just watching my NYC Fire Car running around the layout and noticing the sparking at the rollers and wheels. This is happening on very clean track, but the design of this rig and it's AC motor just give you more "action" at the contact point. I do notice that the Lionel Phantom with the AC motors also has some sparking. Oddly, I don't see that with any of the modern can motored locomotives, at least it's not noticeable like it is with these two. I plopped one of the little AC motor powered
My point is this, every time you see a spark, you can be fairly sure that it's generating some kind of electrical spike along the rails. Having all the engines sitting on powered track with that action can't be as good as having them powered off without any possibility of transient spike damage.
I decided to do a quick test, I have the TMCC #42 Picatinny Arsenal Switcher, and sure enough, it also has visible sparks running around the layout. I repeated the same test with a similar sized locomotive, one of the RMT BEEP's I've converted to TMCC, I didn't see any visible sparks. In the last half of this short clip, you can clearly see the sparking rollers. That's what all your trains on the siding are seeing as well. 
