One option for a power car is to use a caboose for power tethered to the car with speaker/mini FM device. I used a transfer caboose tethered to a Beep or Plymouth because it came with roller pickups on both trucks. There should be plenty of room for your power supply and you won't have to go out and find trucks for your boxcar or gondola.
That should leave plenty of room for your speaker in the other car too.
Beep tethered to transfer caboose with a simple two wire tether to bridge dead spots.
Pete
