Radio Shack's rheostat craps out over 12v, so after smoking up the joint a coupla times, I just switched to a train transformer after reading this article:

http://cs.trains.com/trccs/forums/p/...8.aspx#1328018

and this one:

http://books.google.com/books?id=nnN...esult#PPA22,M1

which led me to getting one of these on page 2 - the 1300:

http://www.hobbylinc.com/prods/una_2.htm

Good Luck!