"The New York City subway system is 108 years old, but it has never faced a disaster as devastating as what we experienced last night," Joseph Lhota, the chairman of the Metropolitan Transit Authority, said in a statement early on Tuesday.

All seven subway tunnels running under the East River from Manhattan to Queens and Brooklyn took in water, and any resulting saltwater damage to the system's electrical components will have to be cleaned - in some cases off-site - before the system can be restored, MTA spokeswoman Diedre Parker said on Tuesday.

At dawn on Tuesday, emergency crews were assessing the damage to tunnels and elevated tracks. Restoring the system is likely to be a gradual process, she said.

"It's really hard to say which areas will come back first," Parker said, adding it will likely be a combination of limited subway and bus service. "It will come back gradually."

The MTA said there was no timetable for getting subways, trains and buses back in operation. The agency dismissed reports on Twitter and other social media sites that subway service would be out for the rest of the week.

About 5.3 million people use the city's subway system on weekdays. The system, which runs around the clock, comprises 21 subway routes linked by 468 stations, and stretches across 660 miles (1,050 km) of track.

The MTA's Metro North Railroad lost power on its suburban Hudson and New Haven lines, while there was flooding in an East River tunnel used by the Long Island Rail Road, the agency said.

The city closed down subway, bus and commuter train systems on Sunday night - a full day before Sandy, one of the biggest storms to ever hit the United States, made landfall on Monday night in neighboring New Jersey.

Sandy was especially imposing because of its wide-ranging winds. The storm brought a record storm surge of almost 14 feet (4.2 meters) to downtown Manhattan, well above the previous record of 10 feet (3 meters) during Hurricane Donna in 1960, the National Weather Service said. (Reporting By Edith Honan; Editing by Eric Beech)

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