San Francisco currently operates the only cable car system in the world. But while today&39's cars are single vehicles, In the old days, cable cars were often actually two separate cars, a dummy--the locomotive--and a trailer. Today, the dummy and the trailer are combined into a single car. These two cars on display in the Cable Car Museum in San Francisco are Sutter Street Dummy No. 46 and Trailer No. 54. No. 54 used to be pulled by horses before the mechanized dummies came along.

SAN FRANCISCO--For almost anyone who's visited Baghdad by the Bay, a ride on one of the city's iconic cable cars is pretty much a must.

What most people don't know, however, is that, once upon a time, these wooden vehicles, which are pulled along their routes by thick, heavy, metal cables, could be found in cities all over the world.

From Paris to Melbourne to Bogota and elsewhere, the cable car was once seen as a fantastic alternative to having horses pull cars of people around. But when electric power made trolley cars efficient, most cities made the switch and abandoned their cable cars. Not San Francisco.

Here, while trolleys, buses, and subways are big parts of the daily commute, cable cars are still the face of the city's public transit system. Whether it's the clanging of a cable car's bell as it crosses Lombard Street, or the lines of people waiting alongside one of the turnarounds, there's little doubt that the system, which operates 40 cars dating as far back as the 1880s, is one of San Francisco's most famous features.

As part of my Road Trip at Home series, I visited the Cable Car Museum yesterday and got a chance to see up close what makes these timeless carriages tick. And up close is right because the building housing the museum is not just a place for cable car lovers to see the history of their beloved vehicles. It's also the home of the last remaining powerhouse and barn for the 40 cable cars that make up the fleet in 2010.

To come near the building is to hear the constant whirring of the cable below the street. And you can even feel the vibrations as you walk across the tracks.

Inside, one of the first things you see are the 12 giant wheels--known as sheaves (pronounced "shiv")--that power the city's three cable car lines.

While it might seem easy to use the word "line" to describe a route, it's also accurate, because a single line of thick metal cable is precisely what pulls along each and every cable car in the city. In fact, underneath the city, four cables pull three routes--the Powell/Mason line, the Powell/Hyde line, and the California line. There's 9,050 feet pulling the Powell cable' 10,500 feet pulling the Mason cable' 15,700 feet pulling the Hyde cable' and 21,500 feet pulling the California cable.

But how do the cars work

In fact, according to Michael Phipps, one of the directors of the Cable Car Museum, the system that runs the cable cars is more or less unchanged since 1887.

It all begins with the sheaves, those huge wheels that are found underneath the building, and which direct the cables out of and into the building. These are the actual cables that pull the cars.

Phipps said that Andrew Hallidie, the man who is largely credited with inventing the cable car, referred to the cables as "endless roadways," since they were essentially continuous wire ropes running the length of each route.

The cables themselves are incredibly strong bundles of metal wires, capable of bending over themselves without breaking, and, of course, of keeping a cable car moving along its line.

Halladie's father had gotten several patents on systems like this, and Halladie himself had figured out how to use the cables as a way of bringing gold ore from mines to mills. And even today, this basic system is in operation at ski lifts the world over.

As for using the system to run cars around San Francisco, Halladie was inspired, so the story goes, by watching an accident when several horses pulling cars were yanked downhill by the car when the line between the driver and the car snapped. The horses had to be destroyed. Halladie seemed to feel, Phipps said, that there was a more humane way to get people around town.

To keep the cars running, the cable is wound around the sheaves in a figure eight, and a system of additional sheaves known as a tension carriage is used to ensure that the lines stay taut at all times. The carriage is on its own set of tracks so it can adjust tension as required by the number of cars on the line and the number of passengers on the cars, Phipps said.

Today, as for decades, the cable runs at a steady 9.5 miles an hour, and when a cable car is locked onto the line, that's how fast it goes, too. Watching a cable car from the street, it's hard to tell what's going on, but in fact, it's quite simple.

There's a device called a grip which drops below the car into a channel and, yes, grips the line. The grip itself works by being dropped down into the channel, and having its jaw pushed open. By pulling back on the jaw, the gripman can close the grip around the cable. When it's fully gripped, the car moves at the speed of the line. By releasing some of the grasp, the gripman (or woman) can reduce the speed, and by letting go altogether, and using the brake, the gripman can stop the car.

This is a standard grip, as is used with all of today&39's cable cars. According to the Cable Car Museum, 'The grip is like a giant pair of pliers that reaches into the channel and clamps onto the moving cable. When the grip has hold of the cable, the car is pulled along with it.'

That's pretty much all the control the gripman has over the car, which is very much at the mercy of the cable as it winds its way under the street.

All this, Phipps said, is why cable cars are capable of going up even very steep hills, even in very wet weather. They have terrific traction, as long as the gripman can successfully grip the cable. And because San Francisco is such a hilly city, that's one reason cable cars have survived the test of time.

As for the brakes, Phipps explained, there are three kinds on board a cable car. There's the regular brakes, which have a foot pedal' track brakes, which shove wood down into the cable channel' and an emergency brake--otherwise known as a "guillotine brake"--which drops into the channel and essentially fuses with the cable. That will stop a car, Phipps said, but will require someone with a torch to burn it out afterward.

1906 As is the case with many things in San Francisco, 1906 was a turning point for the cable car system. That year, of course, was when the city was devastated by a massive earthquake and fire. All around the city, entire neighborhoods were leveled by the quake or the fire, and among the affected industries was the booming cable car business.

Today, the cable cars are run by the San Francisco Municipal Railway (Muni). But in those days, there were cable car lines all over the city, each run by a separate private railway company. Indeed, in order to ensure that competing cars didn't run on their tracks, each company used a different gauge track. And around the city were nine separate car barns and powerhouses. Many were destroyed in 1906 (see video below of the view from a cable car going down Market Street in San Francisco just days before the earthquake).

After the earthquake, some of the lines survived, but others didn't. Those companies that continued operating remained private, but gradually they were consolidated. The last consolidation took place in the 1950s, and from that time on, all cable cars in San Francisco operated out of the single building near Chinatown that today houses the carbarn and powerhouse and the museum.

And when New Zealand finally shut down its last cable car system in 1957, that left San Francisco's as the sole remaining on the planet.

Even San Francisco nearly abandoned the cars. In 1947, a group of politicians and business types tried to shutter the system in favor of newer methods of transit. But thanks to the dogged efforts of a community member named Freidel Klussmann who stood up and demanded that the system be saved.

That November, San Francisco residents voted overwhelmingly to keep the system running, and today, the cable cars are part of the city's charter: only the voters can shut the system down, Phipps said.

And that's fitting. Today, San Francisco operates 28 Powell Street cable cars, each weighing 15,500 pounds, and 12 California Street cars, each coming in at 16,600 pounds. They are, as Phipps points out, America's only moving national historic landmarks.