The diesel engine in a locomotive is not what turns the wheels; rather, the engine turns a generator that powers electric traction motors that drive the wheels. At slow to moderate speeds, a four-axle locomotive with a 3,000-horsepower diesel engine produces more electrical power in its main generator than it can use by itself. This allows for use of a “Mother-Slug” set in certain applications.
In a Mother-Slug set, the Mother is a conventional diesel locomotive that sends its excess electrical power via large cables back to the Slug, which is similar in general appearance to a normal locomotive except that it has only traction motors. A Slug does not have a diesel engine, generator or other components necessary for a "stand alone" locomotive. The Slug contains a large block of ballast to replace the weight of the engine, providing sufficient weight for the Slug to provide traction and pulling ability.
Coupling a Slug to the Mother allows the Mother locomotive’s excess electrical power to drive the Slug’s traction motors essentially for free, using only the power from the Mother. This provides significant fuel savings compared to using two diesel locomotives, as well as producing less exhaust emissions. Mother-Slug sets have been proven to reduce fuel consumption by more than one-third compared to a pair of conventional locomotives, while benefiting the environment by discharging fewer emissions.
G&W’s Canada, Northeast, Pacific, Rail Link and Southern regions currently have 18 Mother-Slug sets in operation.