This is the Thomas the Tank Engine costume I made for my son this past Halloween. The chassis is a Little Tykes wagon, with the pull handle at the back. The body is entirely cardboard, affixed with hot glue. I coated the whole thing in Plasti-Dip (hoping to ruggedize it somewhat), then spray painted it and finished the detail work with duct tape (cut with a drywall knife).
The face is hand-molded air-drying clay, painted with acrylic paint and epoxied to the front of the boiler (a round box purchased at Michael’s). The working funnel contains a thermos filled with dry ice and water, but since the dry ice doesn’t float like steam, I took apart a USB cooling pad for a laptop, and built a housing for it out of cardboard. It’s run off a 9 volt battery, and I used the switch from the cooling pad so it could be turned on and off. The door has a simple Velcro closure, a bell is hung inside (along with a wooden train whistle), and my son used a smartphone app with steam engine sounds to make it chug.
It was a neighborhood hit, and all of his cousins got to take rides too.