Send massage.
The carts are powered by plot. Theorizing about how they work in-universe is fun, but it’s been almost 8 years since that episode aired, and, personally, at no point was I like ‘how is this technicolor equine zooming around in a go-kart with no engine?’ The girls do show up at the end in mechanic jumpsuits, complete with oil stains, implying there are engines involved in the second version of carts.
Now the track, I do question, cause it seemed designed to prompt a collision…
I always assume tech is fueled by magic in Equestria, unless stated or implied otherwise, since that’s how Flim and Flam powered the Cider Squeezy all the way back in season 2.