Have you had this problem? How is it possible that your trains runs great and you bring them to a slow great running stop. Then next time you run them some weeks later , they run like trash even thought the track is clean. Nothing has changed. I had that today , my 2-6-8-0 stalled on most switches and my Riv 0-4-0 would run ether fast or way to slow. Yet these were the engines I ran last and were left just were I stopped them. And am running DC. Here is what I found today with the 2-6-8-0 the tender truck screw had loosened up letting the brass wiper slip to one side making intermittent contact with the brass wheel on the insolated side . it would stall (from a short ) on some switches but not on the rest of the layout. I kept messing with the switch frogs before finally checking the tender, also had a broken wire on slipped wiper. Repaired these. Rivarossi 0-4-0 with tender pickups. Seemed to run slower than normal ,then with a little more power start running fast and would slip the drivers badly . I use a old MCR master 240? controller with a tether , the main box is under the layout and I have to reach somewhat blindly to flip the power switch. However the nudge /pulse switch is right next to it and I turned that on. They both run good now, but was very frustrated fore a hour or so.
Oxidation has changed. Running a locomotive over nickel silver track once takes care of that. Brass is likely to require more friction than one time around.
