Lubricants (graphite for couplers, Aero-car lubes for gears, axles, bearings, etc.) and cleaner (rubbing alcohol). Some paper towels, cotton swabs, old handkerchiefs, etc. are also useful.

To clean engine wheels, lay a paper towel over a powered track, and wet the towel with a little rubbing alcohol. Rail the engine, with one truck on the track, one on the towel, and apply power, while holding the engine in place by hand while the wheels spin.  Then turn it around and clean the other truck the same way.