the girders were hard plastic and the panels were kind of flimsy but thin, stiff sheets of translucent plastic with different parts painted opaque - so that there were solid walls, full windows and partial windows, as well as doors, roofs, etc. like Lancer said, you often ran out of the style of panel you wanted to make the design consistent.
It was the right scale for my cars - HO - but smaller scale than the train set.
the slot car set came with 4 cars - but 3 were grey and one red - BORING!
i couldn't let that stand. it was also hard to tell who was winning if they were the same color.
i left the Corvette red. the Mercedes i left grey. i painted the Jag white with flame decals. and i made the T-bird metallic blue. So cool!