Sand paper really isn't a good thing to clean track with. When railheads are scratched (caused by track rubbers/sand paper etc) the dirt/grime sits in the tiny little dents and scratches (too small for the eye to see) and causes the running problems that locomotives can regularly suffer from.
Rails are also always under attack from something called 'oxidisation' which is when the surface of the metal reacts with the air, which results in a 'film' on the railheads. When you remove this film by using rubbers and anything alse that is abrasive you cause more scratches and dents, which can also speed up the oxidisation process.
The same thing also happens to locomotive wheels, so bear this in mind when cleaning them too.
Poor electrical contact will result in tiny sparks between locomotive wheels and rails, which cause more dirt and even the tiny dents in the track.
This is a very basic explanation so hope it is of some use to someone.