One of a pair of Relieving Bogies which are coupled to both ends of the crane when it is being moved on the mainline.The crane's own suspension is not fit for high-speed operation and these bogies are used to carry most of the weight of the crane during routine movements.Note the dual-braking arrangement as also the hydraulics control unit on the top which is probably used to lift the yellow coupling assembly to reduce the weight of the crane on its own suspension. (Khalid Kagzi)