The northern slope of Fillmore Street between Broadway and Bay climbs at grades of up to 25% -- steep enough that conventional electric streetcars could not manage the ascent reliably. The Market Street Railway's solution was elegant in concept: a looping underground cable connected two steel carriages running on tracks below street-level slots. When a descending car at Broadway connected to the cable and an ascending car at Green did the same, the descending car's electric motor would drive the mechanism, pulling both cars simultaneously. The downhill car's weight assisted the system, making the operation far more efficient than brute electrical force alone. The system was tested on August 5, 1895, and declared "a perfect success from the start." At peak capacity, the Counterbalance could move 8,000 passengers per hour.

Each streetcar serving the route was fitted with a long V-shaped attachment that could rise and fall with grade changes. This device connected to a rod protruding through the street slot via a bolt, coupling the car to the underground cable system. The coupling was straightforward in theory but unforgiving in practice: if the descending car's motorman turned off his motor at any point, both cars stopped. If the coupling failed, there was nothing between a loaded streetcar and the bottom of the hill but gravity and whatever brakes still worked. The system was upgraded over the years -- a heavier cable, redesigned grip, enclosed car bodies with air brakes, and multiple-unit controls for two-car operation. When the Market Street Railway was reorganized as the United Railroads of San Francisco in 1902, the line was split so Counterbalance cars worked exclusively north of Broadway.

On Christmas Eve 1901, car no. 757 slid north down the hill from Broadway without warning. Conductors at the Green Street stop, seeing the car hurtling toward them, shouted warnings that saved most passengers -- but Mary Phelan was killed and nine others injured as no. 757 smashed into a stationary car and continued down the cobblestones until it struck a telegraph pole at Union. Investigation revealed a premature signal bell had prompted the motorman to start before the coupling was secure. More accidents followed with grim regularity. In 1907, ascending car detached from the cable, sending two descending cars plunging downhill "at a terrific pace." Brake failure caused a plunge in 1914. In June 1915, a premature signal caused a two-car ascending train to reverse direction and slam into the train behind it -- conductors herded passengers to the far ends of the cars and opened doors so some could jump to safety. Even sun-softened pavement played a role, credited in 1918 with slowing a runaway car that might otherwise have caused more damage.

The Fillmore Counterbalance made its final run on April 5, 1941. Muni took over the Market Street Railway's operations in 1944 and converted the route to trolleybus service in 1949 as the 22 Fillmore, which still runs today -- though the buses climb the hill using their own electric motors rather than relying on the weight of another vehicle coming down. The underground cable mechanism, the steel carriages, the slot tracks -- all were removed. Nothing physical remains of the Counterbalance today. But the grade remains: Fillmore Street between Broadway and Bay is as steep as it ever was, a reminder of the problem that a generation of transit engineers tried to solve with cables, bolts, and the simple physics of gravity.