COUPLING ROD

A 'coupling rod' or 'side rod' connects the driving wheels of a locomotive. Steam locomotives in particular usually have them, but some diesel and electric locomotives, especially older ones and shunters, also have them. The coupling rods transfer the power to all the wheels.
Locomotion No 1 was the first locomotive to employ coupling rods rather than chains. In the 1930s reliable roller-bearing coupling rods were developed.