Animate train wheels

[jhbmw007]

I'm going to be animating a train and was wondering- is there a way to bone-rig the wheels to come up with a motion similar to this:
http://toons.artie.com/gifs/arg-newtrain-crop.gif

See the two wheels that are connected by the "bar"- is there a way to rig that with bones where the bar stays attached to both wheels? Using angle control bones perhaps? On my train, the wheels may end up being the same size if that will work better- doesn't really matter to me. I just want to be able to turn one wheel and everything else will follow. Is it possible?

[Genete]

viewtopic.php?p=36277#36277

The easy way:
the crankshaft must be a single line (two points). Each point is linked to a rotation bone (small and big wheels) and their rotation connected by angle constraint = 1.

I have no much time now for more explanations maybe along the day if the anme file of the link is not enough.

Cheers!
-G

[jhbmw007]

Thanks Genete. I think I can figure it out- actually the more I think about it, my train will be bouncing back and forth to music, so maybe I'll just use point motion, or combine your method with point motion...