I was working at one of the local factories and they have a crawlspace. A giant entire factory sized crawlspace. a 5' high not enough to stand
so back when the factory was built the maintenance engineers build these recombinant trikes!
it appears that they welded dollies the back of kids bikes and beefed up the axes with pillarblock bearings(see last photo).
here is 2 photos of the basement
and here is a close up of the mechanics for those inclined for a closer look