Cleaning the track and loco wheels is pretty straight-forward, albeit tedious, work.
As for "mass cleaning" the 300+ cars you have, some ideas come to mind:
1) Replace plastic wheels with new metal wheels. My favourite is Proto2000. This could get a little expensive though...
2) A dremel tool with a plastic brush. Get or make a car cradle, and flip a couple of cars at a time upside down and got at 'er. Make sure you do not allow the wheels to spin, as very high RPMs may melt the needlepoint ends of the axels.
3) An ultrasonic cleaner
might work. I don't know, as I have not tried it, but it may be easy enough to do - pop out the wheelsets a couple of dozen at a time, and let them sit in the cleaner.
4) You could do something similar without the ultrasonic part, with a toothbrush and a sink full of soapy water.
Once everything is clean, try the Wahl clipper oil trick to keep everything clean.
Where is your railroad located? Is there a dust problem, or an open ceiling? Humidity/temperature issues? Perhaps there's something bigger than just "crud on the tracks" - it has to come from somewhere...!
Hope that helps.
Andrew