This is an old steam-powered locomotive.
Originally I was just going to rig some wheels and pistons. I wasn't planning to build a train, but somehow it happened anyway.
The model is approximately 20K faces... I tried to strike a good balance between detail and polygon count. If you've got limited computing power, you can delete the bolts, pipes, and beveled edges. That should get you down around 7K faces.
All the mechanical parts are parented to the wheels. I set them up on an extrapolated (linear) IPO curve that cycles @ 1 rotation every 49 frames. I also threw in some basic smoke that starts at frame -20 and goes to frame 100. Just hit "play" and it should work for you.
I didn't do any modeling inside the cab... lots of room for improvement. Also, all the textures are procedural, and I haven't UV mapped anything. Feel free to make it better.