SAN FRANCISCO -- Muni spokesman Judson True did not rule out the possibility of disciplinary action against the operators involved in the derailment of a cable car at Powell and Washington Streets late Sunday that left six people injured as the agency continued its investigation into the incident.

True said the operators should have stayed on the cable car to "call for a push from a truck that could take a couple of minutes to get a truck there."

Investigators estimated that the car was traveling at 15 mph when it crashed, compared to a normal speed of 9.5 mph.

"There are two rules here that are relevant, and we're going to look at them as we go forward," True said. "Both members of the crew must never be absent from the car at the same time. Additionally, the grip man should be at the levers at all times."

True said that the incident, which was reported just before midnight, happened when the car gained downhill momentum without an operator at the controls, causing it to derail at Powell and Washington streets.

Four passengers were injured in the crash, one with a broken leg. The other three sustained minor injuries, True said.

According to Muni investigators, the incident began when the car stopped near Mason and Washington streets.

"At the turn of Mason, right before Washington, there's a slight turn, and it needs enough momentum to carry through that turn," True said. "In this case the cable car didn't have enough momentum and came to a stop on the line where gravity would have taken over."

Passenger Eric Kay said the operators jumped off the car to give it a push after it became stuck at Washington and Mason streets. But once thay got it moving, according to Kay, they couldn't get to the main front brake because a sliding door wouldn't open. Kay said the conductor got off again to try to run ahead to get to the brake but couldn't. Eventually the car careened to a stop.

"I was contemplating jumping off; by the time I thought about it, we were going down the hill so fast and derailed," said Kay.

An initial investigation by the San Francisco Municipal Transportation Authority Monday shed more light on the derailment of a cable car that sent three passengers to the hospital Sunday night, a Muni official said.

The investigation is still active, but True said the accident had no lasting effects to the Muni system and that all cars were able to run as scheduled Monday.

"This is a serious incident, and we'll be taking all necessary steps to prevent it from happening again," True said.

On Monday, tourists were not deterred from riding the cable cars despite the recent accident. Mike Gosztyla of Grove City, Ohio said his family didn't come all this way to miss out on San Francisco's historic cable cars. "I'm doing it for the nostalgia and see what it's like."

Gosztyla added it was a little like a roller coaster, but a little more fun.

While cable cars may be a major tourist attraction for the city, they can be dangerous. Figures from the U.S. Department of Transportation show that mile for mile, cable cars have double the collision injury rate of Muni's buses and more than three times the rate for the light rail system and trolleys. About 20,000 people a day ride the cable cars.