Riders of the A and C subways who took a deep breath yesterday to brace for years of cheek-to-jowl commutes can exhale – in about six to nine months, officials said.

Assessing the prospects for service disruption as a result of a station fire Sunday in Lower Manhattan, officials of New York City Transit shortened their timetable to restore full service for the trains’ 580,000 daily commuters.

The fire, in the Chambers Street station, gutted a locked room housing relays that control the old-fashioned system of switches and signals through which dispatchers maintain a safe flow of trains.

The cause of the fire is still under investigation, police said.

Service on the C line will be completely suspended until sometime between July and October, officials said, and A trains will run at one-third their normal frequency, of up to about eight an hour, until the second week in February, and at 80% of their normal frequency within three months, the president of New York City Transit, Lawrence Reuter, said.

“We will do everything humanly possible to get service back as soon as possible,” he said. “Now it’s a best guess of when we can restore service.”

Transit officials had initially said the disrupted service would continue for as long as five years, raising public ire.

Even if the A and C trains are restored to their normal rush-hour frequency this year, however, it will indeed take from three to five years and cost between $35 million and $60 million to restore the subway to its previous capability, which includes being able to run northbound trains on southbound tracks, officials said. In the meantime, commuters will continue to suffer crowded trains and platforms on affected lines and neighboring lines as well, they warned.

The fire, which began in a shopping cart, possibly lit by a homeless person staving off the winter freeze, quickly engulfed a maze of cables and wires overhead and soon spread to the relays in the room. The 90 relays control switches, which allow trains to change tracks, and signals, which act as traffic lights, along 4,000 feet of track. Officials said workers would cobble a temporary system from extra relays scattered throughout the subway system.

Commuters patiently waited for the A train yesterday at the Chambers Street station, where the odor of smoke and burnt plastic lingered. One rider, Phyllis Fretwell, a Harlem resident who normally commutes on the C and the A trains, recalled a harried adventure journeying to work yesterday on three overcrowded trains.

“When you are trying to get to work on a crowded platform, it’s just not a pretty sight,” she said as she waited for her A train home. “New York is already an easy place to hate, so, great, this gives us one more reason, I guess. And with the weather, it’s just not a lot of fun.”

The work to restore the system in full will bring the technology of the subway line to the MTA’s current standard – “as of 2009”- Mr. Reuter said. The current standard, however, uses technology that is as old as the 100-year-old transit system, relying on the switch and signal system.

It is not computerized.

This summer, transit officials anticipate the debut of the system’s first computer-run subway line, which will replace the mechanical and electrical switch-and-signal system used since the first subway opened in October 1904.The 24-station L line, which has cost $288 million, was chosen to be the first, in part because it is only 10 miles long and does not share tracks with other lines.

By contrast, Mr. Reuter said, the C line intersects with many other lines, and that makes it more difficult to coordinate without all the signals in place. Because fewer riders ride the C train, about 110,000 on an average weekday compared to the 470,000 who ride A trains, its repair was given less priority, Mr. Reuter said.

Earlier reports that the subway repairs would take five years were met with incredulity from the public, especially since the repair was scheduled to take longer than restoring the 1,500 feet of track – only blocks from the Chambers Street station – that were affected by the September 11 attacks. While Sunday afternoon’s fire was the worst mishap to affect the system since the terrorist attacks, the 1 line was repaired in 2001-02 within 13 months.

The fire has highlighted what for many is a vulnerability in the system, which is open to anyone with a $2 MetroCard who can evade the patrols of policemen who regularly roam the platforms and trains.

“It terrifies me that a homeless person with a cart can do this much damage to the New York City subway system,” a commuter, Richard Moschella, said as he boarded an uptown A train.

An assemblywoman from Queens, Catherine Nolan, was one of the elected officials who expressed a similar concern. “If one homeless person can set a fire that, after $50 billion that has been spent over 22 years, can disrupt the entire system, what can one determined terrorist do?” the Democrat said. “It’s a very scary thought.”

Police said they do not suspect the fire was terror-related. “We need to learn what this is before making any further assessments,” a department spokesman, Deputy Commissioner Paul Browne, said.

The Metropolitan Transportation Authority’s proposed five-year capital spending plan for 2005 through 2009 includes $495 million for security, but it does not detail how that money is to be spent, a spokeswoman for the Straphangers Campaign, a subway riders advocacy group, said. “The transit authority should let people know they are working to secure other signal rooms,” the spokeswoman, Neysa Pranger, said.

Police do not believe the fire warrants an overhaul of the security of the subway system, Mr. Browne said. He stressed that police devote more resources to uncovering terrorist plots before they happen, rather than putting officers at every possible terrorist target.