Diesel Locomotive - Multiple-unit Operation

Multiple-unit Operation

Most Diesel locomotives are capable of multiple unit operation (MU) as a means of increasing horsepower and tractive effort when hauling heavy trains. All North American locomotives, including export models, use a standardized AAR electrical control system interconnected by a 27-pin jumper cable between the units. For UK-built locomotives, a number of incompatible control systems are used, but the most common is the Blue Star system, which is electro-pneumatic and fitted to most early diesel classes. A small number of types, typically higher-powered locomotives intended for passenger only work, do not have multiple control systems. In all cases, the electrical control connections made common to all units in a consist are referred to as trainlines. The result is that all locomotives in a consist behave as one in response to the engine driver's control movements.

The ability to MU Diesel-electric locomotives was first introduced in the EMD FT four-unit demonstrator that toured the USA in 1939. At the time, American railroad work rules required that each operating locomotive in a train had to have on board a full crew. EMD circumvented that requirement by joining the individual units of the demonstrator with drawbars instead of conventional knuckle couplers and declaring the combination to be a single locomotive. Electrical interconnections were made so one engine driver could operate the entire consist from the head-end unit. Later on, work rules were amended and the semi-permanent joining of units with drawbars was eliminated in favour of couplers, as servicing had proved to be somewhat cumbersome owing to the total length of the consist (about 200 feet or nearly 61 meters).

In mountainous regions, it is common to interpose helper locomotives in the middle of the train, both to provide the extra power needed to ascend a grade and to limit the amount of stress applied to the draft gear of the car coupled to the head-end power. The helper units in such distributed power configurations are controlled from the lead unit's cab through coded radio signals. Although this is technically not MUing, the behaviour is the same as with physically interconnected units.