It's called "Bridge Builder".
It's cheesy, simple, and an utter black hole for time. I killed about 5 hours with it this afternoon. Basically, you have to build a bridge across a "gap" of water. You have to set up your supports in such a way that they can structurally support the train crossing.
It's got a pretty impressive "physics engine", in my opinion. It takes some real work supporting some of the bridges. ;-)
Oh, yeah, and the more 'points' you have, the more weight. Remember that. ;-)
If you're looking for something to do, you want to try this. It's really small, takes moments to set up and learn, and I promise, you'll enjoy it. ;-)
The homepage is here.
The game itself (all 122kb) is here.
It requires Windows 95/98/00, and DirectX 6+.