How cable cars work

Cable Cars have no engine or motor on the cars themselves. The power source is centralized in the cable car barn and powerhouse at Washington and Mason Streets (also home to the Cable Car Museum). There, powerful electric motors (originally a stationary steam-powered engine) drive giant winding wheels that pull cables through a trench beneath the street, centered under the cable car tracks (that’s what’s in that slot between the tracks).

cablecar-motors-muni.jpgThe four motors and cables driving the cable car system. SF Muni photo.

There are actually four cables, one for the California line, one for Powell Street, and one each for the outer ends of the two Powell lines (Mason and Hyde). Each cable has its own set of winding wheels. The rearmost winding wheel in each set is adjustable. When a cable is new, this rearmost wheel is close to the other winding wheels. As the cable naturally stretches out with use, the wheel is gradually moved back by shop workers to keep constant tension on the cable. The cables are over an inch in diameter, with six steel strands of 19 wires each wrapped around a core of sisal rope.

Each cable car has a mechanical grip (two on the double-end California cars) which latches onto the cable, much like a huge pair of pliers. The gripman (or gripwoman—two women in history have served in that position; we’ll use ‘gripman’ to represent all, uh, grippers) can ‘take’ or ‘drop’ the ‘rope’ (as the cable is called) as needed to start or stop the car. The cables move at a constant 9.5 miles per hour. If a cable car is going faster than that, it’s a sure thing that the car is going downhill and the grip is not holding the rope tightly.

Taking and Dropping the ‘Rope’

At some terminals, you will notice the conductor pulling on a lever in the street. This lifts the cable upward so the grip can grasp it. At other terminals (and at other locations on the system), you will see a noticeable dip in the tracks. This lowers the car, and its grip, to the level of the cable underneath, allowing the grip to grasp the cable.

Among other locations, this happens in both directions where the Powell and California cable lines cross. Adhering to the original cable tradition, the California line, which was built first (in 1878) is entitled to the upper cable. This means California Street cable cars hold onto the “rope” (cable) as they cross Powell Street.

The Powell Street cable cars, by contrast, must drop the cable from the grip before they cross the California Street tracks. If there were no safeguards built into the system, a Powell cable car that held onto the cable too long at this point could pull the lower Powell cable up against the higher California cable and the Powell car’s grip could hit and potentially sever the California Street cable.

To prevent the possibility of this happening, there is an alarm system and mechanism under Powell Street on either side of the California tracks to physically force the Powell cable from the grip if it is held too long. This can damage both the cable and the grip.

Fortunately, because of the skill of the gripmen this almost never happens. Watch a Powell gripman approaching this corner, particularly headed north from Market Street. The gripman has to crest the hill firmly gripping the cable. Then, with one hand clanging the bell to keep crossing automobiles from getting in his way, he throws the grip lever forward with his other hand just in time, then coasts with a clatter across the California tracks.

(A little tower on the southeast corner of California and Powell holds a Muni employee who signals with lights to cable cars on both lines whose turn it is to crest the hill. This is designed to prevent the possibility of cable cars hitting each other at the intersection.)

After the Powell cable car crosses the California tracks, it reaches one of the dips in the pavement, allowing it to “take rope” (reattach to the cable). For Powell cars headed toward Market Street, this happens immediately, before they descend the steep hill to Pine Street. For Wharf-bound Powell cars, however, the tip in the pavement doesn’t come for more than three blocks, where the two Powell lines split, and each gets its own cable. For those three blocks of comparatively gentle downgrade, the Powell cars “freewheel” (run without the cable). This often makes for the fastest part of the trip, since the car is limited to 9.5 miles an hour when it’s holding the cable firmly.

While being a gripman today is a very tough job, consider those on the last all-new cable car line to open, on O’Farrell, Jones, and Hyde Streets in 1891. As the new kid on the block, its cable had to be “inferior” (lower) to older cable lines at every crossing. This meant the gripman on that line (from which a cable car has been preserved and beautifully restored) had to drop the rope no less than 22 times on every round-trip!

Stopping a cable car

Cable cars have three kinds of brakes, all very simple: wheel brakes, track brakes, and an emergency brake. Each wheel has a soft steel shoe that can be pulled tight against the wheel to stop the car. These are crew-activated by foot pedals on both ends of the California cars, and on the front end of the Powell cars. A conductor’s lever on the rear platform activates rear track brakes on Powell cars.

Track brakes are simply pieces of wood located between the wheel sets on the cars. There are four for each car, two feet long each, made of soft Monterey Fir. When the gripman pulls back on the track-brake lever (next to the cable grip lever), the blocks press against the tracks to help stop the car.

The emergency brake is just that. If a cable car gets into a situation where the other brakes won’t stop the car (a very rare situation), the gripman pulls back on the red emergency brake lever. This forces an 18-inch steel wedge into the steel slot between the tracks, stopping the car immediately. (The force is so great that sometimes it takes a cutting torch to get the wedge out of the slot.)

Turning cable cars around

cablecar-turningaround.jpgPowell Street turntable. SF Muni photo.

When single-end Powell Street cable cars reach the ends of the line they are turned around on giant turntables. These are completely mechanical, relying on ball bearings and rollers to move. The cables under the street reverse separately, away from the turntable, wrapping around a large wheel in an underground bunker called a “sheave pit.”

The turntable areas are engineered so that gripmen can simply release the brake and coast onto the turntables. (They have already released the cable at this point.) Once on the turntable, they set the brakes on the car, then climb off, and together with the conductor, reverse the car by either grabbing a stanchion on each end and walking the car around, or using the pipework mounted on all turntables in the past couple of decades to turn the table without touching the cable car itself.

Until the 1970s, passengers were allowed to help the crews turn the cable cars, but this is no longer permitted. Also until that time, passengers could jump onto the cable car as soon as the gripman began coasting toward the turntable, getting a free spin on the turntable as well as guaranteeing the seat of their choice for the upcoming ride. This too is now forbidden for safety reasons, with boarding is strictly controlled by a queue.

The California Street cars were built with grips and brakes at each end precisely so turntables would not be necessary. The gripman operates the car through a switch at the end of the line. After applying the brakes, the gripman and conductor switch places, and the cable car goes in the other direction. The switch at the terminal has a spring on it to keep it aligned with the departure track, thus preventing possible derailments.