While drop-in decoders are a great idea in theory, in practice they just don't work all that well without some modification - a coat of solder on the pickup pads, for example, to make sure the connections are tight, and maybe even hard-wiring the motor.
The right way to do this is what Fox Valley does: you have a light board made to spec with the loco that has a socket for a plug-in decoder. Problem solved. If only everyone else would adopt this system.
John C.