A cold snap that set at least one Long Island weather record delayed trains, ruptured water lines and pipes, and created face-numbing outdoor working conditions Tuesday.

Another frigid commute in single-digit temperatures is on tap Wednesday morning.

The National Weather Service and health officials renewed warnings that prolonged exposure to the cold can cause hypothermia and frostbite.

Tuesday's low was 7 degrees at Brookhaven National Laboratory in Upton and Long Island MacArthur Airport in Islip.

The record low for the date is 3.5 degrees, set in Upton in 1980, the weather service said. In Islip, where records date back 30 years, yesterday's low shattered the previous mark of 14 degrees in 1986.

"Overall, I would say it was a bitter, cold day across Long Island," said David Stark, a weather service meteorologist in Upton.

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Relief is in sight. The forecast calls for temperatures to reach the low 20s Wednesday and climb back above freezing again Thursday.

The cold snap came courtesy of a polar vortex -- a massive low-pressure system that hit the Northeast with a vengeance. Tuesday's cold was in stark contrast to balmy Monday, when high temperatures across the Island were in the mid-50s.

Tuesday's freeze caused 10-minute delays on Long Island Rail Road trains into and out of Penn Station during the morning commute, the agency said.

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In the afternoon, the Suffolk County Water Authority reported four water main breaks caused by the cold -- in Farmingville, Bay Shore, Lindenhurst and East Islip.

"Intense cold is the worst thing for the pipes," said authority spokesman Tim Motz.

Artic temperatures also wreaked havoc with household plumbing in some areas.

"We're seeing a lot of burst pipes," said Kristina Merritts, office manager at Miracle Plumbing and Heating in Copiague.

Pothole-repair season has arrived sooner than usual because Monday's warm spell was followed so swiftly by intense cold, said Eileen Peters, a state Department of Transportation spokeswoman.

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Thawing and freezing cycles allow water to first fill cracks in the pavement and then expand when it turns to ice. That renders the road vulnerable to impact from heavy trucks, Peters said.

Area hospitals, meanwhile, reported no cases of frostbite or hypothermia Tuesday.

"I think people are being smarter," said Colleen Valdini, a spokeswoman at Good Samaritan Hospital Medical Center in West Islip.

Electricians like Chuck Tekverk, who was hoisted about 20 feet in the air running new cable for a traffic signal in Commack, wisely dressed in layers.

"The winds are always blowing a little colder up top," said Tekverk, 48, of Kings Park. "This is one of the colder days, but not my coldest."

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With Gary Dymski,

Joan Gralla, Patricia Kitchen and Ridgely Ochs