Yeah...an operating G-Scale train is a little pricy, but the tracks are close to the right size and you might be able to find some at a Hobbytown (or hobby shop of your choice) for less cost (time) than what it would take to build.

Another idea would be one of those large scale battery powered holiday trains for the base of a Christmas tree. I've seen them in sizes that could be big enough for the 1/18th scale.

A final idea could be a forced perspective of an elevated train. This could use a much smaller and likely less expensive train/tracks to provide the train. The drawback to this is that the Joes could not actually be seen on the train without a bit of Photoshop work.

Monetarily, the project can get costly. No matter what direction you decide, good luck. I look forward to seeing the result when your finished.

Bayer