Alex, The decoder was made first to fit the GP-35 and GP-9. The SD40-2 was made afterward and should of been made to fit the TCS (which means the same board as the Geeps). A bit much to ask TCS or Digitrax to make a decoder just for the SD unit. David I thik the SD40-2 will be the most popular z scale loco for some time, so digitrax or TCS would do well to have a specific decoder for this loco, I cant wait for MTL or AZL to have DCC factory installed decoders. Mike.