There was a similar discussion on an N scale forum about this, and track/wheel cleaning in general. Using something stronger than alcohol was suggested, such as "Goo Gone" cleaner, which is citrus based. I tried some on the wheels of an AF hopper (which I had cleaned a few weeks earlier with alcohol) and here was the result: 
As you can see, it really did the trick!
I like the fact that it:
1. Evaporates quickly
2. Leaves no residue
3. Has a rather pleasant smell
4. Doesn't attack plastics (as far as I can tell)
Since it is more expensive than alcohol, I use it for initial "deep cleaning", then use alcohol for more routine maintenance.
Mark in Oregon
