I installed the electronics package about 9 months ago? Long before I had access to the cab interior parts. Having never seen them I had no idea how much space the assembly would take.
The command boards are in the middle between the motors and are out of the way. The sound board would only fit if I tried stacking it on top of the command boards which I really did not want to do. I don't like to butcher the Williams connectors and I make up an adaptor to tie the two motors together and feed the ERR board. This uses RC car harness parts and makes up a wad of wire about the size of a ping pong ball. I had considered putting the sound board there in a vertical mount but decided there was plenty of space in the front of the shell. Technically there is plenty of space, just not enough to use the Lionel parts the way Lionel intended them to be used.
The actual cab area is pretty small and I have no problems cutting it up even farther to do what I want to do. Kitbashing plastic parts isn't that big a deal to me and it may be easier than moving the electronics around since those are actually working.
