To expand on Vic's comment about a paper towel placed across the track, that is what I do. And it can make a mess depending on how you approach it. One of the worst things that can happen is you get the cleaner fluid (whatever you choose) on your hand and then it gets on car bodies, leaving fingerprints or worse. To prevent this, I use an eye dropper to apply the fluid to just an inch or so of the towel on the track. Roll one truck of the car back and forth (best on a curve so as to get maximum contact) over the part of the towel with fluid, then roll forward to clean the other truck. Meanwhile, the first truck is on the dry part of the towel. So the wheels are dry when you are done. And it only takes a couple drops of fluid. I will typically run a train to the section of mainline I use for cleaning, and just clean one car after another, moving the towel a bit when it gets dirty. The towel only has to be moist to clean, not soaking wet. It takes less than 30 seconds per car.
Hope this helps.
PS I recommend using only metal wheels. They don't pick up and deposit the crud plastic ones do. I know this for sure after my most recent cleaning marathon, the metal wheels just weren;t nearly as dirty as the plastic ones.