I found this awesome coaster sandbox game the other day... it's still Early Access, but it's amazing. It has custom coasters, custom trains, custom terrain, custom flat rides, and pretty much custom everything. It's still missing a few track types and all, but for what it is now it's a great purchase. It costs $30.
Some coasters I made in it as examples:
Wing Coaster:
Entire Park:
Kingda Ka on steroids:
Ghetto Looper:
Floorless Coaster:
Mine Train:
Good Ol' Woodie:
...and these aren't very good. You'd be amazed to see what the community has done with particle generators, lighting effects, and the like. It's awesome.