To connect to the cable system, a long V-shaped attachment, free to rise and fall with grade changes, was added to each streetcar serving the route.

)[3] According to a Market Street Railway official, it would be impossible to stop a car running downhill using the onboard brakes if it was not attached to the cable, as the wheels would skid faster than they could roll.

[6] The line was served by the smallest streetcars in San Francisco: California-type "dinkies" with a single truck, built by the Hammond Car Company.

When MSRy was reorganized to incorporate several private rivals as the United Railroads of San Francisco (URR) in 1902, the line was split so that Counterbalance cars exclusively worked north of Broadway, and regular streetcars operated south of there.

[10] However, the tunnel was delayed and cancelled in September 1913;[11] instead, URR rebuilt fourteen of the Fillmore Hill dinkies and operated them in two-car trainsets to improve capacity over the route; the original open bodies were enclosed, air brakes were added, and multiple unit controls were installed.

757, which was carrying approximately fifteen passengers, suddenly slid north down the hill without warning from its starting position at Broadway.

757 continued downward after the collision, jumping its tracks and running on the cobblestone street until it collided with a telegraph pole at Union.

[6] Mayor-elect Eugene Schmitz witnessed the accident from his home and called the offices of the Market Street Railway, asking them to send medical help.

[13] A subsequent court hearing found that while the conductor failed to couple the car to the cable properly, he was not faulted because the timer had rung the signal bell prematurely, prompting the motorman to start operation before the Counterbalance was ready.

[21] In November 1921, a two-story house at 2924-2926 Fillmore was jolted off its foundations after a runaway downhill Counterbalance car left the tracks at Union and smashed into the building.