My wife makes all kinds of comments about my train-on-the-carpet room, none complimentary. Great chart exculpating the trains from ‘firebug’ status.....
I seem to recall people having melting issues with smoking cabooses.
I too use to lay a screwdriver over the Super-o track to add to the excitement running my old GG-1!
I remember being about 5 years old, wrapping bare wire around the plugs on a nightlight, plugging it in, and watching it fire across the room! Every light in the house went out! 
Still alive at 55!
I don’t think you could get a fire from the sparks off the wheels and rollers unless a flammable substance was there. Think of all the carpet centrals that do no not catch on fire. If it were a problem we would have been hearing about it all our lifetimes. Spill some alcohol or something on the carpet than all bets are off. Sounds like the fires are likely from dead shorts. My postwar type layout uses outdoor type carpet.
