Hi
look, what I've done for a sound job for the little Bachmann Davenport.
http://www.youtube.com/watch?v=rJjNry70mZM
The sound decoder is a Zimo MX 645r, between the Bachmann pcb and the weight are 4 capacitors with 4.7 mF parallel installed, the sound project I've done myself.
The capacitors gives the safety of a free run without electric pickup for 4 inches in a slow speed. The capacitors are connected to the capacitors gain of the decoder.
The run is on another club members layout
Have fun
Heinz D