The power failures that shut down the Lexington Avenue subway line twice yesterday – including at the start of the morning rush – may be linked to a 2-by-3 foot breach at the base of a manhole below Park Avenue South, transit officials said.

It is unclear who dug the hole directly above the southbound subway tracks at the 33rd Street station, a spokesman for New York City Transit, Charles Seaton, said. Transit officials believe water and salt seeped through the hole and onto cables running along the roof of the tunnel. An investigation is ongoing, Mr. Seaton said.

Yesterday was the second time in as many months that riders coped with a major service disruption as they did after a fire at the Chambers Street station in January that crippled A and C line service for several weeks.

New York City Transit restored full service on the Lexington Avenue line before the evening commute, but not before riders – who thought their troubles were behind them – faced another power outage, albeit considerably shorter and less disruptive.

At 12:42 p.m., less than two hours after transit officials said full service on the 4, 5, and 6 trains had been restored, power to a signal box and to the control tower at Grand Central Terminal failed when workers short-circuited the power flow while changing cables at the 33rd Street station.

Power was restored by 3:10 p.m. and full service on the Lexington Avenue line, which serves about 350,000 passengers during daily rush hours, resumed shortly thereafter, Mr. Seaton said.

For transit work crews, it was the end of an eventful day that began when power to the signals along the Lexington line stopped flowing at 7:20 a.m. Electricity to the third rail was not affected, so motormen were able to drive the trains to the next station before evacuating the cars, Mr. Seaton said.

Service on the nos. 4 and 6 lines was completely suspended. Soon after the first power outage, the no. 5 train from 149th Street and Grand Concourse in the Bronx was rerouted along the no. 2 line on the West Side until returning to its normal track at Nevins Street in Brooklyn. The result was heavy congestion and slow-moving trains on the Broadway line.

At Union Square station yesterday morning, Ana Reiselman learned of the service disruption when “people were coming up the stairs saying, ‘they’re not running, good luck.’ ” Ms. Reiselman, a 23-year-old Greenpoint resident, said trying “not to be one of the angry ones” yesterday was as difficult as getting to her job at Grand Central Terminal via the Q train to Times Square.

“I try not to be frustrated because it’s a waste of positive energy,” she said. Still, Ms. Reiselman said she is angry at the MTA after recent fare increases for weekly and monthly passes. “They’re charging more money and the trains aren’t working. I don’t know what I’m paying more for,” she said.

West Side riders complained of slow moving trains as displaced no. 5 trains were routed onto the Broadway line tracks used by the nos. 2 and 3 trains.

One subway rider, Tom Acitelli, who is 28 and lives at the Upper West Side, said it took 40 minutes to go from 96th Street to Times Square. A service disruption, he said, is the worst way to start one’s day.

“I’d rather take a cold shower than have a subway disruption,” he said.

Frustrations reached a boiling point at Times Square, where displaced passengers seeking to head east on the shuttle service to Grand Central Terminal intersected on overcrowded trains.

Though conductors tried to smooth over tensions with announcements on the intercom, most of their words fell on the deaf ears of angry passengers whose morning commutes had already been ruined.