difference between uncoupled and coupoled locomotive

In coupled locomotives, a driving wheel is driven by the locomotive's pistons and the driving wheels are all coupled together with side rods or coupling rods. Normally one pair is directly driven by the main rod which is connected to the end of the piston rod and power is transmitted to the others through the side rods. eg. Steam Locomotives

In uncoupled locomotives, the driving wheels are directly driven by the traction motors. Coupling rods are not used. eg. Diesel & Electric Locomotives.

Locomotives are of two type coupled and uncoupled.If two or more pairs of wheels are coupled together to increase the adhesive force between the wheels and the track,it is called coupled otherwise uncoupled.