LGB New Orleans Streetcar, LGB Stainz, Aristo-Craft Lil' Critter Switcher, LGB Santa Handcar.The black sludge is probably from arcing between rails and wheels/pickups.
At least that's what I have in my dry but dusty location.
Wiping down the rails or if they are corroded getting them shiny helps
Metal shavings is a lot more concerning. A bit more inspecting necessary.
What locomotives are they?
Today's theory is that my Christmas track circle is too small for the wheelbase of the LBG Streetcar. Although it seems to run OK, I bet the wheels are shaving off brass.I have found some wheels too far apart on the axles, esp some USA Trains axle sets. I place them in my vise and 'squeeze' then together slightly. I do not even have to remove them from the trucks using the vice. How to tell? Place an axle on a piece of track and try to move sideways. Some wheel sets do not move at all, LGB axles move a lot. Of course a wheel guage would be helpful here as well.