My dad and I built something coaster-like when I was younger. Our backyard is hilly and has woods at the edge. What we did was we took a length of cable two smaller pieces and created two loops of cable around two trees, one uphill from the other, and used turnbuckles to connect the long piece to the loops and tighten it. We strung a pulley onto the long piece and attached a rope holding a plank of wood to it to make a cart. Add in a platform courtesy of redneck engineering and we had a coaster-like-zipline-play equipment-fun for drunk adults at neighborhood parties-thing. It wasn't a coaster, I don't count it in my count, but it was fun. It's no longer standing because the uphill tree supporting the platform and half the device had to be cut down due to an invasive species that killed it. But it was a ton of fun, and the part at the bottom of the hill was in the woods so gliding down the hill a few inches from the ground into the Forest was a great deal of fun, especially at night.
