Scottmac, I don't know if this will help you or not. I also don't know how to get at the wipers on the back of the wheels either. However, I own several Porters, both 0-4-0 and 0-4-2, which are notoriously finicky when it comes to power pickup. What I do is lay the locomotive upside-down in a foam cradle and apply power to the wheels using a 9-volt battery, This is enough to turn the wheels slowly. Then I take a cotton swab with isopropyl alcohol and hold it onto the back of each wheel as it is turning. I do this several times to each wheel. You would be amazed at the amount of gunk that comes off each wheel. The above procedure usually solves my electrical pickup problems. Just so you know, I generally only have to do this approximately once or twice a year. FYI, I also own a Whitcomb diesel but have not had any problems with it so far. Hope this helps.