Do the trains break down? Can you kaboom trains into each other?
No. You have some control over your vehicles, you can tell them to start, stop, turn around, and return to the depot, but other than sending them to the depot you generally only use them when you've messed up your signalling system and caused some sort of gridlock. Trains won't run into each other but they won't go through each other either so if you have two trains facing each other on the same track they're not going anywhere.
So the game is basically done now?
It's as done as it's going to get. They're using what they learned from making this game to make a similar sequel with a wider scope.
Oh, is there a Sandbox mode?
The game is pretty much just a Sandbox mode. You start on a clean world, either randomly generated or created from an external height map, with some small scattered towns and non-functioning industries and you're free to connect them up however you want.
But keep in mind that just because you set up some really nice transportation system it doesn't mean that the people are going to use it. If it doesn't get them where they want to go in a timely manner your shiny new transportation system may be doomed to a slow bankruptcy.