LOS ANGELES, JULY 13 -- Los Angeles is celebrating this weekend the rebirth of rail mass transit in the world's most automobile-addicted city, and the expensive experiment in social engineering is having a difficult start.

As officials made final preparations today for Saturday's opening of the first commuter train here in 29 years, a fire in a tunnel being constructed for an underground section of the proposed 150-mile Metro system closed Highway 101 and created day-long havoc for downtown automobile traffic.

For years, old-timers here have complained about the scrapping of the Red Car trolleys, which made their last run from Long Beach on the southern edge of Los Angeles County to downtown Los Angeles in 1961.

Fictionalized books and films have suggested that the rail system was killed by a conspiracy of automobile and oil companies, although most experts say the Red Cars could not compete with the convenience and flexibility of automobile and bus travel.

With highways now congested even in off-peak hours and stringent new air quality rules imminent, local officials have successfully revived the rail system, with modifications, as one way to wean Southern Californians from their automobiles.

The scheduled debut Saturday of shiny new 230-passenger rail cars on the 22-mile Blue Line from Long Beach to Los Angeles marks the first step in what is expected to be a long and uncertain fight against ingrained commuting habits and budget problems.

"With the opening of the Metro Blue Line, public rail transit permanently comes back to Los Angeles," said Los Angeles Mayor Tom Bradley, a relentlessly optimistic advocate of the new mass-transit system.

Today, Bradley was forced to spend much of his time wrestling with the Metro tunnel fire. The blaze, whose cause was being investigated, caused no serious injuries but snarled highway traffic so badly that police recommended that office workers be sent home early.

The $877 million Blue Line opening Saturday is above ground and generally has been welcomed by commuters and businesses, but the more expensive underground portions of the rest of the system have drawn criticism, particularly from affluent northwestern suburbs that expect little use from Metro.

Plans call for a $10 billion rail complex that could link Los Angeles with cities as far away as Cerritos on the Orange County border.

To encourage Blue Line ridership, Los Angeles County Transportation Commission officials plan fireworks and music Saturday and will let everyone ride free until July 30. The fare then will be $1.10, about half the cost of a bus ride along the same route.

Unscrupulous passengers may continue to ride free because the line is to run on a honor system, with no gates and only random checks of riders who fail to put money in the ticket machines.

The commission has been much more attentive to security along the route through prime gang territory, signing a $10-million-a-year contract with the sheriff's department for special transit detail.

In their prime, the Red Cars made the Long Beach to Los Angeles run in 36 minutes. The Blue Line, because of frequent stops and the crush of automobile cross traffic, is expected to take 54 minutes.

The transportation commission is leery of ridership predictions, calling them "not an exact science," but suggests only 12,000 riders a day by 1991, a pittance compared with the county's 4 million automobile commuters.

Yet optimists abound. "What the opening of the Pasadena Freeway symbolized for the beginnings of a freeway system," county supervisor Ed Edelman said, "the Blue Line . . . will symbolize for mass transit."