Unrailed!

Versus adds a competitive edge to things, Quick tasks you with getting to the next station as fast as possible, while Sandbox lets you play with unlocked upgrades without risk of failure. That's about all there is to it — it's a very straightforward game with a neat central idea. The only real trouble is that, due to a lack of real depth, this probably won't last you very long. If there was a little more to it, it'd be easier to recommend, but it's certainly worth a quick trip.