This idea is a combination of this:
with this
The first picture is way too expensive. The second is less expensive, but I think one could improve it's stability AND perhaps further reduce the price as follows: Remove the PVC rail from one side. Rotate the "tie" on the remaining rail and stick the end opposite the PVC rail into the ground. Looks more like the first picture now but with a very low cost, /round/ PVC "rail" instead of the I-beam. The increase in number of connections to the group (vs just the few 4x4 risers) would increase stability and the savings on PVC rail and risers would pay for the extra concrete. Or perhaps even allow one to replace the risers with those metal fence posts which are simply pounded into the ground. On the "rail" end, just drill a hole in the bottom of the PVC and set the rail over the top of the post. In any case, the risers would be quite numerous, spaced at some even distance... say X inches apart. Perhaps 24" or 36" The train would follow the rail with 4 scooter wheels arrayed at 10, 2, 4, and 8 o'clock...
Ah, you say, but how will you keep the train from falling over? It would stay upright as follows: Each car or engine in the train would be Y long, where Y > X. E.g. longer than the spacing of the risers. And on the bottom, each has a set of guides which rub left and right of the risers. These would be made of steel, but coated with a slippery surface e.g. teflon to reduce friction. Keep in mind, the train is setup such that as much weight as possible is /below/ the rail (batteries, motor, frame, etc...) and the load is placed as low as possible. The riders would straddle the rail, feet on either side, and would recline or lay forward as on a bicycle. This greatly reduces the forces required to keep things upright.
If further friction reduction is required, the guides would be replaced with a track or belt system like a tank or treadmill. This could also be helpful in the case of a powered train, because the risers become cogs to be grabbed by the tank/belt when moving uphill at high rates of incline. At worst, a low end scooter wheel could be added near the top of each riser.