running the rails for fun and exploration of abandoned railways that no one but a couple train operators ever got to look at

unlike places in the eastern US and the midwest, there isn't any call to get these hundreds of miles of desert railways converted to hiking or biking trails, and they aren't in the region where they could be added to a cross country hiking/biking route. And I doubt enough people ever would use them recreationally to make it worth anyone's time and effort to do anything with them... but this, building a car, and going for a unique ride. 

The source article goes into a bit of detail on the cart construction, probably from the you tubers links, but basically it's 2×3 framing with a plywood deck. 

Some extra support is added for the motor mount and for the seating location. It uses slightly longer go-kart axles to accommodate the width of the railroad, and a small 6HP gas engine with a single gear to power the rear axle. 

There are no brakes and the real hack here is [Ryan]’s custom wheels. He found that steel or cast wheels were not particularly comfortable on long journeys so after a few attempts he has come up with a home-built polyurethane wheel which is cast in a mold around a steel go-cart wheel and then trimmed on a lathe.

https://hackaday.com/2023/02/25/cart-cruises-abandoned-california-rail/