After removing the locomotives wheels I place enough 91% alcohol in a small dessert bowl to cover the wheels and place the wheels in the alcohol and let soak while I clean the pickup wipers and inspect the drive train I then clean the wheels with a old tooth brush and shop paper towel.
You may wonder: But,Larry isn’t that going to the extreme?[:O]
Absolutely but,now not only are my wheels clean so are the wipers.[:D]
I appreciate all your suggestions and generous input. My xacto knife did the trick, wheels clean and shiny, loco bolts down the track like a new one. Dstarr and Lion, thank you! -Rob
I use 2 methods to clean locomotive wheels, one is the good old Kaydee brand brass wire brush wheel cleaner that you connect to track power and use to polish the wheels and remove that crud. For unpowered tender wheels and diesels with stubborn crud, I use my rechargeable dremel motor tool with a wire brush in it on the low speed setting. I use my finger to keep the tender wheel sets from spinning to fast, and a set of test leads to power the diesel engine to rotate its wheels while I polish them up. Going over to 100% metal wheels in your rolling stock will help keep locomotive wheels cleaner. Mike the Aspie
I have a small wire brush that I chuck in my Dremel. I find that if I wipe the rails down with a piece of cotton tee shirt, I don’t have to clean the wheels very often. It also helps that I’ve changed over from plastic to metal wheels…plastic seems to drag and deposit that crud all over the place 
All my locomotives are DC, so this is pretty easy for me. I clamp the chassis upside down in a vise. I use alligator clips to run power to the motor.
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Once the wheels are turning, I use a bamboo skewer to get the crud off. I can dip the skewer into alcohol if it is nasty crud. The bamboo will mot hurt the drivers.
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-Kevin
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