I recently purchased a used G scale streetcar (United Traction Co. model).  Both axle gears are broken.  I have been looking for instructions for removing the wheels from the axles to make sure that this is easily done before I purchase replacement geared axles.  Do the wheels just pull off of the axles or is there some specific mechanism for removing the wheels?

Octopamine,

I don't know of any available formal instructions, I have not personally done it on the streetcar, but the wheels are press fit onto the axle splines.  I place the wheel to be removed on top of a small vise between the jaws with the axle tip facing up, without tightening the vise against the axle. I then use a wooden dowel to tap the axle down to the wheel face, once down I use a thin tip awl or thin punch and tap the axle the rest of the way out. 
Just go easy tapping on the ends so as to avoid damage to the wheels.

Best of luck with your repair.

Bill

Bill,
Thanks for the note.  I'll try your method.

I had the same problem with THREE trolleys.  Six axles, five cracked gears!  I 3D printed replacements in both PLA and ABS.  I put the file up on Tinkercad.  Look for "gear" and it should show up.  Else write to me directly [rfriedma at pacbelldotnet] and I can show you the file and the results, since I can't figure out how to put photos up on this board.