Trainz

On my first computer, Auran's Trainz Simulator worked but my machine was dangerously close to the minimum system requirements.
The newer machine didn't like it for some reason when I was running Win2K, but now that I'm running XP and have more RAM the game works really well.
In fact, when the simulator came out (2001) my current computer would have been considered an ultimate gaming machine that was worth a bajillion dollars.

Here are some screenshots:
Don't tell me that isn't Mr. T in that van.

The speed limit is very low when you're running in heavy snow.

Love my smoking AlCo FPA2 locomotives.

I'm going to have to poke around the interwebs for some patches this game needs.
Apparently, Auran is out of business so I'm also SOL for adding some D&H engines to my fleet.
Operations are MUCH easier in DCC (model train) mode versus messing with the complicated braking systems of the "real" train system, but I'll be switching back next time.

The funnest part is being able to swing several cameras around and take photos from nearly any angle without getting rained upon.
IF I wanted to get crazy, you can also use the game to design a model railroad layout down to the smallest detail.