San Francisco's Municipal Railway has a peculiar problem -- its F- Embarcadero streetcar line, which runs from the Castro to Fisherman's Wharf, is too successful for its own good.

The line, which uses vintage streetcars from around the United States, and a few from Italy, is packed with passengers all day and half the night. On Saturdays and Sundays it's even worse.

"Some passengers tell me that as many as five cars passed them up before they could get on," Charley Robinson, a regular streetcar driver, said the other afternoon.

His car had standing room only when he left Fisherman's Wharf and swung onto the Embarcadero. By the time he got to the Ferry Building, the customers were packed in like the proverbial sardines.

It was hot and stuffy inside. Kids were crying and riders -- most of them tourists -- were wondering where they were. Robinson, a cheerful man, was peppered with questions. "How do you get to Chinatown? Which way is the train station? Do you go to Golden Gate Park?"

"We are basically overwhelmed," said Muni executive director Michael Burns. "The F line is a victim of its own success," agreed Rick Laubscher, president of the nonprofit Market Street Railway, a group of rail advocates that fought to get the streetcar line going.

When the F cars started running to Fisherman's Wharf four years ago, they carried good loads, but since then, the economy picked up, the Ferry Building was refurbished, and the once-shabby Embarcadero became one of the city's premier boulevards. Word about the streetcar line got out.

"We were told this ride was really nice," said Matthew Liddy, a tourist from England, who waited with his wife, Clara, at Beach and Stockton streets, across from Pier 39 Wednesday afternoon. There was a three-hour wait for the cable car, so they opted for the streetcar. "They are charming," Clara said.

They waited 10 minutes or so: One car stopped and loaded what looked like a hundred people inside. "No more," the operator said, and closed the door. The next car passed with a ring of the bell and a wave of the operator's hand. Too full to stop.

The Liddys peered down Beach Street, waiting for another car to come. They were dressed for summer, and in the San Francisco chill, the charm was beginning to wear off.

"In their wildest dreams, nobody predicted the F line would be this successful," said Laubscher. "People should be delighted."

Some of them are. On a quiet day, the ride along the waterfront in a brightly painted old streetcar is a delight, with views of the bay, sailboats and big ships. Ticket price: $1.25.

But the cars are so crowded that regular riders who work along the Embarcadero or use the F to connect with BART or the Muni Metro subway can't get aboard, much less find a seat.

"We get the locals mixed in with the tourists," said Robinson, the streetcar driver, "and they get pretty mad if we have to pass them up."

"This line is slow and crowded," said a woman who works in a law office near the waterfront and uses the F car to go back and forth to the Embarcadero BART station every day. "These tourists take it because they like it. I take it because I have no choice."

Some Muni regulars quote Yogi Berra, the baseball player who said of a popular restaurant: "Nobody goes there any more. It's too crowded."

20,000 passengers a day

The F streetcar line, which is 4 miles long and cost $55 million to build, carries an estimated 20,000 passengers on an average day, many more in the summer tourist season.

That's as many people as all three cable car lines combined, and only 8, 000 fewer than the entire Caltrain Peninsula train line, which stretches from San Francisco to Gilroy. San Francisco's F line has 1,000 more riders than Santa Clara Valley's entire light rail system. "That's tremendous for a single line," said Randy Rentschler, spokesman for the Metropolitan Transportation Commission.

"It's carrying twice the projections (of ridership)," Burns said. "It's a tremendous success. We are proud of what it does."

However, the Muni doesn't have enough streetcars to meet the demand, and on many days the rail service is supplemented with buses. "I saw a passenger refuse to board a bus the other day," said Al Baccari, former head of the Fisherman's Wharf Merchants Association. "It was hysterical. He wanted a streetcar, he said, not a bus."

To run the service, the Muni has 17 streamlined Presidents Conference Committee cars, which look modern but are mostly more than 50 years old. Though nearly all of them came from Philadelphia, they are painted in colors of various street railways from around the country. There are also nine Italian trams that ran in Milan.

In addition, the railway has six historic cars, including streetcar No. 1, the Muni's first ever vehicle, which began service on the long-vanished A- Geary line back in 1912. Car No. 1 is a real museum piece, the oldest operating streetcar in regular service in the United States.

It's out there nearly every day, still doing the job, though the car needs paint and looks a little shabby, and crews are careful not to let the antique car get too packed. "It's like playing Barry Bonds every day," said Laubscher. "Can you imagine playing Barry Bonds when he's 91 years old?"

11 new trains on the way

The Muni, meanwhile, is working on a solution to the car shortage. Eleven PCC cars that used to run in Newark, N.J., have been purchased at a cost of about $15,000 each. The cars, built in the 1940s, ran in subway service in New Jersey and are said to be in good shape.

However, they have to be modified and refurbished for service in San Francisco, and Burns says the Newark cars won't be on the San Francisco streets until next year.

In the meantime, the Muni has acquired a used Belgian tram and is looking for other options, including rebuilding some retired San Francisco cars that have been stored for years.

There are big plans to extend the streetcar service south along the Embarcadero, perhaps beyond SBC Park, and perhaps later north and west to Fort Mason.

The southern Embarcadero service -- to be called the E line -- could start next year, Burns said.