There is an article on the Ottawa Valley GRS website that shows how to remove the track power wiring from a Bachmann 4-4-0 American.
Place the locomotive upside-down in a soft cradle or in the top of the Styrofoam packaging it came in, taking care not to damage the whistle.
Remove the 8 screws holding the cover on the bottom of the motor block. Place them in a small container so they do not get lost or mixed up with others.
Unsolder the four track power wires from the bottom of the axle bushings. Shrink wrap the ends of the wires and tuck them back in out of the way. This prevents the locomotive from feeding battery power back into the tracks.
Re-install the cover on the bottom of the motor block using the eight screws in the small container.
That completes the modifications to the locomotive. Set it aside for now.
Place the tender upside-down in the cradle.
Remove the four screws that hold the copper, axle wipers and brass, loop connectors to the bolsters. Remove the axle wipers. This prevents the tender from feeding battery power back into the tracks.
Re-install the 4 screws back over the loop connectors. The screws fasten to the frames that hold the brake shoes.
Another article on installing a battery and Revolution receiver with steam sounds in a Bachmann Annie can be used as a guide to do the same for the American locomotive.