Abstract: The apparatus includes a master control unit associated with a first locomotive and a slave control unit associated with a second locomotive. Each of the control units is identically constructed to include an electronic processor and memory. The memory includes a program which is also identical for both units. Each of the units is connected to at least a portion of a train line extending between the locomotives, and inter-unit communications are accomplished over only a single spare line within the train line. Under control of the program the control units detect the setting of the throttle in the first locomotive, and in response thereto the control units operate the throttle of the second locomotive so that the combined operation of the first and second locomotives functions at an optimum power output-to-fuel consumption ratio within a predetermined range of power levels for each setting of the throttle in the first locomotive.