Note:  Do not rely on this information. It is very old.

Coupling Rod

Coupling Rod, in Engineering, is a bar of wrought-iron or steel joining corresponding points on wheels of equal size in such a way that it will always move parallel to itself, and will therefore transfer motion of the one to the other. In a locomotive it is necessary that the driving-wheels shall be sufficiently loaded to produce the requisite amount of friction between the tires and the rails. With a single pair of driving-wheels this may be difficult to effect, especially when the engine has to draw a heavy train. Hence the advantage of coupling the driving-wheels with another pair of equal size. This in effect means doubling the load on the driving-wheels.