I only clean track in areas where I've been applying scenic materials. There are some areas that have never been cleaned in the fifteen-or-so years that the layout has been here.
My layout is in a separate, unheated, but well insulated room in the basement. The floor is unpainted concrete, and there's a suspended ceiling installed. I occasionally vacuum the cobwebs and dust, and sometimes the floor too. This isn't to say that the track doesn't get dirty, though. I was running the loco, shown below, one day, with a
fairly heavy train. I was walking alongside as it entered a grade, and as more and more cars rolled onto the grade, the loco's wheels began to slip, until finally, its forward progress was halted. As the wheels spun desperately, I noticed something being thrown from them. I cut the power, then lifted the loco from the track, to inspect the wheels. Most were partially clean and polished, but there were places where the crud was so thick, the wheel flanges were non-existent.

This is a remotored Athearn, with sintered iron wheels, supposedly notorious for picking up dirt. However, if you look closely at the truck beneath the cab, you'll notice the Tomar pick-up shoes, which undoubtedly kept the old girl supplied with power despite the dirty wheels. This loco has also been re-geared, and is extemely slow-running, and I wonder if that has anything to do with the wheels getting so bad. None of my other Athearn switchers, with the original gears, suffer from this problem.
The only cars that seem to get dirty wheels are the cabooses, all of which have been fitted with Kadee trucks. I only clean them if the crud gets so thick that there's no wheel flange to keep them on the track.

Wayne