Rigs of Rods

Rigs of Rods is described as a sandbox-style vehicle simulation. It has a very impressive physics engine (and I really can't stress this enough, check the video below!) that makes it possible to drive everything from trucks and trains to ships and aircraft. Different parts of each vehicle can interact independently of each other, so stags, beams and wheels can react naturally to the environment. Moreover, there's buoyancy, advanced flight models, and more. I usually don't post videos, but this pretty much requires one. Check it in action below. The game is of course highly moddable, so construction of new vehicles is possible. There is also a repository of content to browse from. And this is probably the only place in the gaming world where you can "drive" a building crane. :)
If you are at all interested in big vehicles, this should be on your list.

Drive trains, and make sure they don't derail.