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Aug 31, 2023

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It’s strange but true: Diesel-electric locomotives have streetcar roots. With diesel locomotives rapidly replacing steam locomotives in the years after World War II, it’s easy to imagine diesels as a natural evolution of the steam locomotive. The fact is, there was almost no transfer of technology. The melodious steam whistle was replaced by a blaring air horn. In place of a rope-pulled clapper with a brass bell, there was a pneumatic ringer for a steel bell. The 6ET (Engine & Tender) brake system morphed into the 6BL (Branch Line) brake system, still lacking a self-lapping automatic brake valve and the vital brake pipe pressure maintaining feature.

The real technology transfer started years earlier for the origins of the diesel-electric locomotive are rooted in streetcars, not steam engines. Early diesel-electric locomotives reflected the legacy of streetcar development in several ways, some of which are present on all new locomotives to this day.

Let’s begin with DC series wound motors. Engineers and scientists working on streetcar development saw the clear advantages of DC motors for propulsion, specifically, the characteristics of high torque at low rotational speeds. Power at low speed matched beautifully with the needs of railroad operations for starting trains and offered a big advantage over steam. While the armatures may have gotten bigger (and the insulating material certainly did again and again), this DC motor design remained almost the exclusive traction motor until a century later when variable-speed AC motors became practical.

As they pondered the correct voltage and current for streetcar propulsion, electrical engineers landed on the progression of a series circuit followed by a series-parallel circuit and then a full parallel cabling arrangement for the traction motors to manage the appropriate levels of current (amps) for starting and voltage for higher speeds. This all-important series-parallel power circuit was present in new locomotives until the 1980s — when continued increases in traction alternator output made this motor arrangement obsolete.

Early streetcar designers struggled with transmitting power from the DC series motor to the wheels. Awkward arrangements used belts, gears, and axle-mounted armatures. All proved unsatisfactory because there was no reasonable way to keep the motor aligned with the wheelset while traversing the track. In the streetcar solution suggested by inventor Frank Sprague, the traction motor hangs off the axle. Sprague called this “wheelbarrow suspension” and it eliminated the alignment issue. It remains in use worldwide.

The selection of carbon materials for the electrical brushes necessary for commutation of DC motors was a stroke of genius that solved the primary DC motor reliability issue early in the 2oth century. All locomotives used this technology until the introduction of AC traction motors in the 1990s. In fact, a whole industry manufacturing carbon brushes for industrial America was spawned to support DC motors. Ever heard of the National Carbon Company, later acquired by Union Carbide? Even today, more than half of all North American locomotives use carbon brushes.

During the streetcar era, speed control was achieved with six to eight notches on a hand-operated rheostat. Similarly, the eight-notch throttle control of internal combustion engines is a characteristic of almost every diesel-electric locomotive built in North America. (The half-throttle positions on General Electric locomotives of the 1960s were a rare exception.)

And lastly, all the important aspects of multiple-unit (M.U.) control still used for diesel-electric locomotives were introduced for streetcar use by Frank Sprague in his Oct. 16, 1900, patent on this subject. For good reason, many people associate the success of diesel-electric locomotives with multiple control. The impact of MU controls on freight train length is undeniable. Aggregating control of a second, third, fourth, and even fifth locomotive under the control of a single operator has yielded terrific productivity gains since the steam era. But it’s worth remembering Frank Sprague had multiple unit control patented and running on Chicago’s Southside Elevated Railroad by 1897. While the initial MU cable had only five connectors, the precedent for MU operations involving multiple rail vehicles with DC traction motors was in place.

And so, streetcars laid the foundation for diesel-electric locomotives. While the complexity of diesel locomotives long ago eclipsed streetcars, today’s light rail vehicles are head-to-head competitors for the application of modern technology.

Don Graab spent most of his 39-year railroad career in locomotive maintenance-related areas. He retired from Norfolk Southern as vice president-mechanical.

Like diesel locomotives? Thank a trolley!