FreeGames/BridgeBuilder

Link: http://www.bridgebuilder-game.com/ (also has several other good games) http://www.chroniclogic.com/index.htm?pontifex2.htm (has many bridge builder games) http://www.armadillorun.com/ (only similar)
Genre: Sandbox.
Platform: Win32, Linux, Mac.
Rico factor: One.
Multiplayer: Nope.

So, let's see. Physics engines. What can we do with them?
Why, we can connect things together and stack things on top of other things.

Brilliant! Let's call the first things 'girders' and the second things 'trains'.

So, yeah. You build a bridge and you try to stop it collapsing under its own weight, and then you run a train or two over it.

This is all made much much easier by having seen bridges in real life and knowing kinda how they're supposed to look.

They're made slightly game-like by making you span different shaped areas, giving you different materials and limiting your budget.

But basically, it's like playing with lego and then watching it all fall down.

First link is actually quite hard. Second link is shareware, but much prettier - though has very few free levels. Third link is insane.