I purchased most of my freight cars off ebay. Some were obviously very old and quite well used. There was an unbelievable amount of gunk built up on some of the wheels, in some cases I changed out the trucks altogether, sometimes I had to scrape off the junk with an exacto knife then clean them up with alcohol.
Even though my engines run smooth, I clean my track every couple weeks or so; to avoid this very thing. I use a good system of cloth and a small wood block and rubbing alcohol, going over the track and moving the cloth as the dirt accumulates. It only takes me a few minutes since I have done it so many times I have gotten quite good at it.
Dust from the basement and airborne particles, I believe, are the culprits. Sometimes when I turn on the shop vac very tiny dust that gets through the filters, I can see, blows out and I am sure that ends up on the track as well.
Hi LASM, Glad you affirmed the gunk build up on the wheels as it can, as you say, get quite bad. The easiest way I've found, in the past at least, to clean the wheels on the various cars is to hold them or lay them in a foam work cradle and use a variable speed dremel tool at a fairly slow speed with a wire wheel in it and let it rotate one wheel of an axel while holding slight tention on the other wheel to create a bit of drag which helps te wire wheel to brush the wheel off good.
Woodland Scenics says their Tidy Track, 'Roto Wheel Cleaner' while meant to clean the Loco's Drivers, also can be used, non-electrified to clean car wheels by roaling the the car back and forth on the fiber pads which you can also apply some cleaning solution to first but I have yet to try it on the cars. It seems to work fairly well on the locomotives.
Their Hand wand with the various pads works well too, especially with the pads that have groves cut in them so they ride the rails without wandering, I use the more abrasive pads and when those get a bit of dirt build up on them I use a hand held wire brush to clean them off so it's ready to go again.
What's nice about the wand is that the pivoting head can be somewhat locked in position for straight track or the base with the pads can be rotated 180* so it feely pivots for curved track and the design of the handle and how it attaches to the head allows you to easily move it through the tightest scenery or into tunnel portals with ease yet still hold downward pressure on it.