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 MU 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.