Metro

It’s the LIRR-arrrgh!

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The LIRR will derail trips for thousands of riders this weekend as it essentially shuts down service to work on a massive signal and switch overhaul.

Passengers on the nation’s largest commuter railroad are being told to allow for 70 additional minutes of travel time with only three trains hourly operating in either direction between Penn and Jamaica stations from 12:01 a.m. tomorrow until 11:59 p.m. Sunday.

That’s a stunning two-thirds reduction in regular weekend service.

Also, no trains will run to Atlantic Terminal in Brooklyn. Only the Port Washington branch won’t lose service, and extra trains will run there.

“This is totally inconvenient! I have things to do in the city all weekend and now I have limited time to get in and out of here — if my train is running at all,” said Francesca Cracchiolo, 17, of Plainview, LI, at Penn Station yesterday.

Most riders said they had few alternatives.

“It’ll cost way too much to get a cab into town, and it’ll take two times as long to drive,” said James Glick, 30, a banker from Wantagh, LI.

“Everyone is upset, and they’re going to hit us with a fare hike in January,” he added.

Officials need to nearly shut down the railroad to replace the ancient signal and switch system responsible for the fire in late August that halted most service for almost a week.

The current system of manually operated cords and levers will be replaced by computer track switching.

“It’s bittersweet,” said longtime leverman John Viesti. “Right now the system is a bunch of roads, like a Jenga puzzle, only with a finite amount of movement.”

Riders will face a second hellish weekend on Nov. 6-7.

Average LIRR weekend ridership through Jamaica is more than 156,000.

The new, $65 million microprocessor system is designed to prevent future disastrous shutdowns.

“Customers should use the LIRR for essential business travel only during these two weekends,” said railroad president Helena Williams.

“Customers . . . should consider travel on the Port Washington branch or other travel alternatives.”

To get into the city from Long Island, most riders will have to get off at Jamaica and take the E subway line the rest of the way to Penn Station.

Williams reminded riders to check the MTA Web site for train times and information.

“Our simple message is to know before you go,” she said, and not just show up at a station.

The few running trains will bypass some stations in Queens, including Hollis, Kew Gardens and Forest Hills.

tom.namako@nypost.com