When the 2nd loco is attached to the main loco, both are connected by MU (Multiple unit) operation cables. These cables carry the control signals from the first loco to the other. This way, only the first loco is manned while the 2nd one is remotely controlled from the 1st loco. In cases where the second loco is attached at the other end of the train ( like bankers), the rear loco is either controlled by radio ( This system is called Locotrol ) or both Loco pilots co-ordinate with each other using hand-signals or wireless communication.