I just saw this video, it seems to me that it'd be pretty easy to make a rainbow rail, or some other stupid shape for a backyard setup. I guess it'd be harder with thicker pipes but you could use multiple skinny pipes if you like wide rails. Is there a reason people don't do this other than the effort required to make a support structure that fits a curved object and you probably wouldn't have enough speed for any kind up up rail? You could make S rails.
https://www.newschoolers.com/videos/watch/965051/Easy-PVC-Pipe-Bending--How-To-DIY-