I might have to do this in two cars. A box car to to have all the electronics and maybe a flatbed car to hold the show car (Hot wheel). I might even have to run a tender between the two cars. Or have two separate cars (one flatbed and one box car) with two sets of roller pick up one for each car.
Does the gondola with HotWheel have other onboard electronics (lights, whatever) or is this a strictly sound affair? If it's just sound then why not skip the tether from the boxcar-to-gondola. If the speaker is placed in the boxcar - maybe on the end of the boxcar closest to the gondola - then no tether is needed. Steam engines with a tether between locomotive-tender have the speaker in the tender even though the sounds are coming from the locomotive - can't tell the difference.
Also wrt the speaker, perhaps you can find one of those animated sound boxcars that has broken electronics and hence get it cheap off eBay or a train swap meet or put a posting on Want-to-Buy. In addition to installed powered trucks, the chassis ought to have a grille already punched out to mount a speaker probably facing downward to the track. I'd think that would save a lot of time rather than grinding on a steel chassis to make a speaker vent.
And how is the sound activated? Any time there's track power? And/or is there an on/off mechanical switch? Or maybe an IR sensor on the boxcar triggers when it passes some point on the layout and the sound is meant to play for 1 minute or whatever? Since this is in the planning stage, this is the time to, uhh, plan.
