Thanks folks.
As for the audio, I used the same board as Steve. I've since found a source for self-recordable ones for about $12/ea, so in any future projects, I'll have a cheaper source. The three boards illustrated are the sound board, the ERR MiniCommander 2 TMCC board, and the custom power supply and trigger board for the sound card from the TMCC control.
I was looking for a novelty project for shows, and when I saw Steve's creation, I thought it was really cool, so I set about making one. Since our displays are all command, it required command capability. I did get a bit carried away with the lighting, but what the heck. 
And, you can't have a hot dog running around on your tracks without a driver, so I had to put a couple people in the cab. 
The original Wienermobile was $6 on eBay, so that part of the raw materials was cheap. The whole project cost a bit more than $100 for the parts, and with my new cheaper audio card source, it would be closer to $70. Of course, I don't count the parts box and all the labor actually getting it to work. 
