I recently went through my stock of powered motherboards I had gathered during the half-price parts sale. One was clearly defective, that will teach me not to test the parts from Lionel before putting them in stock! 
The rest of the powered motherboards seem to function properly, hopefully they don't go bad in the drawers!
BRUK, that buffer will usually solve the problem of the RS4 board taking too much current from the serial data and the Cruise Commander running away, that's why I use it. The really odd part is, sometimes I need it, and sometimes I don't. Usually, I've needed it with the Cruise Commander M, apparently there is a greater load imposed on the serial data for that board than for the plain Cruise Commander.
