At first glance, it's hard to tell what makes a cable car move. Stop anywhere along the tracks and you might get a clue. If it's quiet enough, you'll hear the cables singing as they travel through a slot below the street's surface and between the rails. The cars hook onto the cable to move and let go when they want to stop.
There's one cable for each cable car line, drawn over large pulleys called "sheaves." The two in this picture are in the Cable Car Museum. They run the California and Mason lines. Because the cable can stretch up to 100 feet during its lifetime, the equipment you see in the background adjusts for the stretch to keep the tension constant.
