OK here it is - almost everything you ever wanted to know about rigging a steam engine. :eek:
First a picture. I like pictures:
Some things to note:
The wheel connecting rods on any x-6-x steam loco MUST be in two bits to allow the wheels to move in the Z direction over uneven track, or it will bend.
The middle drive wheels do not have any flange - it would b****r the track and wheels totally!
The mid point of the piston stroke should point towards the centre of the driven wheel to which the con rod is connected, so you get even drive.
The two sides should be 90 degrees out of phase - remember steam cylinders are double action, effectively a “1 stroke” engine!
Here’s the blend file, I will answer any questions you may have. You should know from this how to rig any wheel combination and how to rig the body parts. I have included a curved bit of track. To drive the rig, simply move the “Empty-control” object (hint - it’s on one of the undisplayed layers) or just press Play to see it operating. The valve gear is pretty straight forward once you have got your head around this bit, so that’s your homework. :ba:
Enjoy! 
steamtrain.blend (1.28 MB)
Cheers, Clock.
PS. play close attention to bone parenting and driver values…