LOS ANGELES -- Commuters tired of battling their way to work on car-choked freeways in the nation's dirtiest air took their first trains to work yesterday and found standing room only.

The Blue Line trains, the first in the city, officially opened for business, running at 10-minute intervals on an $877 million, 19-mile rail service between the financial district of Los Angeles and the nearby port of Long Beach.

Each two-car train can carry up to 450 passengers, all traveling free for the first two weeks to encourage people to leave their cars. Officials of the Los Angeles County Transportation Commission have scaled back predictions for the opening period from 30,000 riders a day to 7,000. But the first-day count was about 10,000, officials said.

"I thoroughly enjoyed my ride," said Alan Lamsbert, a bank vice president. "It was relaxing. You're lucky if you can get up to 20 miles an hour on the freeways in the rush hours and I read my newspaper fully for the first time for years."

When the free period ends, travelers will pay a basic fare of $1.10 for a one-way journey, whether they make the full 55-minute journey or leave the train at one of the 20 stations along the route.

If the line is a success, Los Angeles will build a $5 billion, 150-mile rail service over the next 20 years.