Multiple-unit train control (MU), is a method of simultaneously controlling all the traction equipment in a train from a single location, whether it is a Multiple unit comprising a number of self-powered passenger cars or a set of locomotives. Usually, the control of the consisit is vested in the cabin of leading loco, irrespective of the number of locos MUed.
When a locomotive is carried alongside load(like wagons/coaches), it is considered a 'dead loco'; such locomotives will be dysfunctional & its resources are not utilised in haulage. Generally 'dead locos' are seen attached at the front end of the consist.