Neat idea.
A local minister here built one, but his was steel 55 gallon drums. Just welded the tongue, hitch, and axle in place. Painted each car a different color. He used an old riding lawn mower for the "engine."
It was used several times at local churches, and he once trailered it to the Shriner's hospital to give free rides in the parking lot. Made lots of smiles with that train.