The actual cause of brush material build-up in the commutator slots in motors is the brushes themselves. They are too soft. The basic construction of all those early motors was basically the same, copper commutator plates, armature, magnets housing, etc., so it's not likely to be any of those things.
My original Atlas/RoCo locomotives from the early - mid seventies were terrible for that build up and fire rings when they were new. However, after all these years, they now run great and the commutator slots stay unplugged. I can only assume the brushes have hardened a bit over time.
Also, I know max and others have devised excellent methods to improve, drastically, the old Trix K4, especially current pickup. I have a thing about keeping old things original, however, so I just disassemble everything and completely clean and polish every surface possibly involved with current transfer and they run fine after that. An important, possibly overlooked surface is the drawbar stud on the trailing truck on which the spring wire from the tender rides. Even though it's chemically blackened Zamac, it still gets dirty. Make sure there is plenty of tension on the spring wire, too, so it bears against the stud reliably.
I did have to run a wire directly from the front tender truck pickup to the motor on one of mine because, somehow once, while I was running it, a short occurred on one of my Treble-O-Lectric switches and it burnt off part of the spring wire immediately (I saw it glow bright red) so it no longer contacted the drawbar stud. Even ignoring the destruction of the spring wire, there was a drastic improvement in continuity but I don't think I had cleaned the stud yet before that happened.
Doug
