Shay engines are steam powered engines which use vertically aligned pistons on the side, which turn a crank shaft with gears shafts attached to the wheels.  They were typically used for logging and mining operations where the curves tended to be tighter, the grades steaper, and the track more irregualar.  


The pistons and crankshaft move, powered by a 9v technic motor.  While not obvious from the picture, the boiler is offset to the side of the engine, so there is room for the steam cylinders on the side - which is typical of real shay engines.  

Shay with Tender

Since the engine needs power  for the 9v motor, power comes from a tender which doubles as a tool and supply car.

Real Shay

A real shay engine located near Portland, Oregon.