hi there, just to add to this, minitrix sells a very ingenius and easy to use wheel cleaner. it consists of 2 sets of miniature wire brushes that sit in a cradle. you place this cradle onto a piece of energized track from where it takes the power (dcc or analogue) and passes it up to the 2 lines of wire brushes that are at about similiar spacing to n gauge rails.
you the place the loco onto the brushes which are now energized and hey presto the loco wheels spin, and the brushes that the wheels are sitting in clean them as they spin!
have a lookie
here:-
minitrix wheels cleaner
best thing i ever bought!.
any loco wheels that do pick up current though and are not powered u need to use a method like described above!