the british rail class 13 was formed in 1965. the tinsley marshalling yard needed more powerful shunters to move vehicles around. this was provided by permanently coupling together two class 08 shunters as a ‘master and slave’ (or ‘cow and calf’) unit. the second of the two units had its cab taken off. both units were ballasted to improve traction. at first the units were coupled cab-to-cab but it was easier to couple master nose to slave cab. the thinking behind adopting this dual locomotive design is that a larger, rigid locomotive could not be used due to the risk of grounding on the hump.