This piece is part of the Sidewalk Talk series “15 Innovations That Shaped the Modern City.”

Horses did the dirty work of mass transit in the late 19th century—and it was literally dirty, as trolley-pulling horses strafed city streets with manure. The electric traction motor not only put horses out of the trolley business, it wound up altering the course of urban transportation. Pioneered by Thomas Edison protégé (and later rival) Frank Sprague, the electric traction motor powered streetcars, elevators, and railways, and made subways feasible, enabling cities to expand rapidly. As historian Frank Rowsome writes in The Birth of Electric Traction, Sprague “almost single-handedly wired electricity into the second industrial revolution as a basic source of power and transportation.”

The origins

The world’s first subway opened in London in 1863 and eased congestion on its city streets, but it had a dirty downside. Black soot from its coal-powered steam engines polluted the air and the lungs of thousands of daily passengers. While efficient train scheduling helped the subway win rapid adoption—carrying 9.5 million passengers in its first year—its biggest drawback was “smoke-filled stations and carriages” that forced city officials to drill openings in streets and remove glazing in station roofs.

In 1882, Franklin Sprague, a 25-year-old naval officer on leave in London, rode the underground Metropolitan Railway, analyzed its flaws, and envisioned a cleaner, smoother subway powered by electric motors. An inveterate tinkerer since he was a young boy, Sprague had designed just such a motor. During two years at sea following his graduation from the Naval Academy, he filled notebooks with detailed sketches of his perfect motor.

The breakthrough

After a brief, unhappy stint working for Thomas Edison, who insisted that Sprague work on incandescent light projects that neither interested nor challenged him, Sprague struck out on his own and built his first electric motor. He displayed it at the 1884 Philadelphia Exposition, where it caused a sensation, since it produced no sparks and could run at constant speeds whether it was hauling loads of 40 or 400 pounds. Even Edison was wowed by his ex-employee’s invention, proclaiming, “His is the only true motor.”

Sprague’s motor impressed influential engineers and investors, and netted Sprague’s young company many orders. But Sprague knew his traction motor had bigger potential. All he needed was an opportunity to persuade politicians and wealthy patrons that electric traction could power mass transit on a large scale. He got his chance in 1887 in Richmond, Virginia.

Backed by a group of New York investors, the city hired Sprague to build the Richmond Union Passenger Railway, a 12-mile electric trolley system on hilly terrain, in a mere 90 days. The steep 10-percent grade wasn’t Sprague’s only challenge; he also battled typhoid fever during the construction. But Sprague pulled off the project. He mounted his traction motors on the trolley’s axle. An overhead wire supplied power. A switch inside the car controlled speed based on resistance of the motor’s field winding.

Sprague’s most crucial test in Richmond came during a demonstration for Boston industrialist Henry Whitney. One midnight in July 1888, Sprague cued up 22 streetcars and, with the wave of a lantern, sent them slowly climbing a hill until they disappeared from view. Whitney was sold. He championed Sprague to electrify the entire Boston streetcar system.

The impact

Cities soon rushed to electrify their streetcars and establish the overhead wires on which they depended. Just 18 months after the Richmond railway opened, 110 cities began using electricity to power trolley lines. Thanks to his invention of multiple-unit control, Sprague helped create the Chicago El train (1895), as well as the Boston (1897) and New York City (1904) subway systems. Rather than large, heavy locomotives hauling the dead weight of train cars, multiple-unit control trains equipped each car with its own motor, allowing for greater speed and acceleration.

By ushering in electric mass transit, traction motors accelerated city expansion into suburbs. Motorized trolleys tripled the speed of horse-drawn trolleys from five to 15 miles per hour, making open land beyond the city accessible to mass commuting for the first time. The shape of cities began to change from dense urban centers to a star-shaped spatial structure, with suburban streets radiating from new transit corridors. For less-affluent workers forced to live in cramped tenements around factories, streetcars made possible “ribbon communities” (typically 12 to 15 miles outside a city), helping to create a new commuter class.

“Far more than the steam railroads and horse cars ever had, trolleys stimulated the rapid growth of suburbs,” writes Rowsome.

The future

Sprague’s impact remains strong more than a century after his innovations. High-speed (or bullet) trains like Japan’s Shinkansen, which debuted in 1964, still operate on distributed traction systems. They use electric multiple unit systems, which are similar to multiple-unit controls but incorporate regenerative braking and power converters for greater speed. Many countries in Europe and Asia connect major cities with bullet trains, which can reach speeds of 250 miles per hour or more. China has 14,000 miles of high-speed railway, about two-thirds of the high-speed rail in the world.

When it comes to the future of mass transit by rail, much rides on the evolution of magnetic-levitation. Frictionless maglev trains have already set world speed records; in 2017, a Japanese maglev hit 374 miles per hour. But due to huge infrastructure costs, only three maglev systems are currently operating globally: in Japan, China and South Korea. Tesla founder Elon Musk’s futuristic train concept, the Hyperloop, which combines maglev tech and a low-pressure transit tube, promises even higher-speed long-distance commuting between urban hubs, potentially reaching a whopping 746 mph.

Even if that goal were realized, finding room for such a massive system within dense urban areas presents a big challenges, which might explain Musk’s recent exploration into underground tunnels.