All The Currently Available, Lego Compatible, Train Wheel Sizes
I'm surprised that nobody I know of hasn't already assembled together a pic like this one yet: All the currently available sizes of Lego train wheels and drivers in one photo, for comparison of their relative diameters.
Represented in the pic are:
Lego's wheels, of course, but also...
Big Ben Bricks:
...and Shupp's Brick Train Stuff:
Shupp's stuff is 3D printed, but it is all very high quality plastic with good fit tolerances, and frankly, he has so many sizes, shapes, and spoke styles that aren't shown here, that you must go visit his Shapeways page and look at what he's got to offer. There are 33 (!) different train wheel types there as of this writing.
Thanks for supplying us train heads with all the sorely needed variety, Shupp. :)