How is this game? How has it come along? Has development sputtered out?

How is it comparing to say RRT3 and OpenTTD?

As someone who has played at least 250+ hours of TF I can say that a lot of the mechanics of the game were smoothed out, but it's never going to be comparable to RRT3 or Open TTD because it's not a Tycoon type game. If it was marketed as one it shouldn't have been. There's no stock market and there's no competition, the entire challenge comes from building a working train/truck/bus/tram system to ship people and cargo from place to place in a timely manner and hopefully make a profit while doing so.

There are hundreds of mods out there but the development on TF has pretty much ended because the company is working on their next game Transport Fever which will be adding ships and planes and a campaign of some sort to an improved game engine while keeping the same basic gameplay as TF.

If you have any more specific questions I can try to answer them.

Do the trains break down? Can you kaboom trains into each other?

So the game is basically done now?

Oh, is there a Sandbox mode?

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. 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. 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.