Never knew such things existed! Where do you find such a contraption and what happens when a train comes?
Do a search for speeder motor cars for sale that will get you started. Narcoa web site has a for sale section. Most of the cars that are ready and certified for track use will average $5,000. Kinda cheap really for this type of hobby.
It's illegal to trespass on any railroad property much less put a speeder on a track and run. Organizations such as NARCOA (one of many) negotiate with mainly small, shortline railroad companies to run the members equipment on. Insurance is required (abt $100 yr), on the average $100 track fees (varies with mileage), and sign a waiver of railroad not responsible for personal injury.
All speeders are required to have narrow band radios that can operate on host railroad frequencies. These are used to communicate between all railcars and the railroad. A railroad employee sometimes will lead the speeders in his Hi-rail truck where traffic is active. (Extra cost)
On our excursion, the owner and CEO that leases the rail line had his own speeder with us. He was also the locomotive engineer so there was no other movements on the track other than our group.