Unrailed is a co-op survival game in which you must keep a train going for as long as possible. It's a novel twist on the genre that sees you and three other players protecting a locomotive rather than yourselves. It even has the usual trappings of survival games, such as procedural level generation and crafting, but it keeps it all very simple.
When you start, the train moves very slowly, and you and your partners must mine for wood and iron in order to build pieces of track to keep things moving. It's pretty easy, although there are other things to think about. Your engine will heat up over time, and you'll need to throw water over it to prevent fires from breaking out. You also need to build bridges across water rivers and watch out for enemies that'll steal your resources.
Once your train reaches the next station, you get the chance to upgrade your engine, the carriages, or buy new ones. Upgrades let you craft faster, build higher stacks of items, slow the train down, and more. Once you've made your choice, the game carries on, amping up the difficulty until it's game over. That's on the default Endless mode, anyway — there are a couple more ways to play.
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.