Hi Nick,
I would avoid using WD40 because as NZKen mentioned above, it could damage the plastic. Are there any hobbyshops close to where you live? They should have some type of light machine oil with a needle applicator, specially suited for lubricating your locos. I wish I knew more but haven't done that in so long myself, I might only confuse you more...
As for using an eraser, is it a so-called "bright-boy" type specially designed for cleaning track? Standard erasers may be too soft, where they would not only fail to get all of the gunk, but also leave a residue of rubber.
My layout is still under construction, and my previous one was in a basement where the nickel-silver track didn't get dirty that much, so my experience on this topic is also somewhat limited.