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Commuters waiting at the Long Island Rail Road station in Jamaica, Queens, on Wednesday.Credit Andrew Renneisen for The New York Times

Long Island Rail Road trains going into and out of Pennsylvania Station were suspended on Wednesday morning for about two hours because of a power failure, snarling service for thousands of commuters.

The Port Washington branch was shut down entirely around 8 a.m., closing one of the main mass transit routes to the United States Open tennis tournament in Queens.

For hours before that, starting around 5 a.m., trains from across Long Island were being held east of the East River tunnels, where a power failure had caused signal trouble.

The Long Island Rail Road said around 9:20 a.m. that it was in the process of restoring service to Manhattan, almost two hours after it had gone down. Officials said they were setting up shuttle service at 15-minute intervals between Jamaica, Queens, and Pennsylvania Station, and at longer intervals from other stations in Queens. Limited service on the Port Washington branch was also restored.

But many commuters had already gotten their fill of clogged platforms and stalled trains. On Twitter, they griped about platforms packed three-deep with waiting riders and angry crowds spreading to the city’s subway system as the railroad diverted commuters.

Jaime Renwick said in an email that she had gotten on a train from Wantagh to Atlantic Terminal in Brooklyn around 7:45 a.m., but that two hours later she was still stuck in Jamaica, one of the region’s busiest transit hubs. Ms. Renwick, who is eight months pregnant, said she had decided to turn around and head back home to avoid the risk of “getting bombarded on subways,” which she noted are very crowded even on a good day.

The Long Island Rail Road was generally reliable, she said, but added that the trouble on Wednesday “seems to be a perfect storm of power outage, increase in commuters due to school starting and humid weather. Not fun!”

The cancellations, coming about a month after a series of electrical problems crippled New Jersey Transit service, were another reminder of the toll that aging infrastructure has taken on some of the nation’s busiest sections of track.

Officials said New York City Transit was honoring Long Island Rail Road tickets on the E, 7, 2 and 3 subway lines, and suggested that fans traveling to the United States Open take the No. 7 train.

But those accommodations threatened to add stress to a system that is already overburdened and has faced its own share of complaints recently about big crowds and delays. Commuters waiting for subways complained of “bedlam” and said they were already hours late to work.

“Will the E train EVER arrive?” one rider asked on Twitter around 8:30 a.m.