Back in the day I always used meths to clean the track. ALways did the trick.
There are model railway "rubbers" available too which do the trick
If you mean the grey has gone a bit yellow then this is more of an issue - you
can use hydrogen peroxide and UV light but this can cause the plastic to become
brittle. I have a large amount of this track and for the yellowed bits its just
something I live with
The peroxide solution may cause the metal to rust too