When the 6 a.m. Amtrak train pulled out of New York's Pennsylvania Station yesterday and then turned around and went back instead of going to Washington, one thought ran through the minds of many antiwar demonstrators aboard: It was Karl Rove's fault.

"This has Rove's fingerprints all over it," passenger Julie Finch said of President Bush's top White House political strategist, who is often a target of Bush opponents. "He's trying to keep people from showing up to protest the war in Iraq."

It turned out that electrical problems, caused by a beam that fell onto wires on a track in New Jersey, caused Amtrak to suspend southbound rail service out of New York for nearly four hours yesterday morning.

Twelve Amtrak trains between New York and Washington were disrupted and one was canceled, affecting the travel plans of at least hundreds and possibly more than 1,000 protesters, organizers said.

Metro riders encountered delays as well because trains on the Blue and Yellow lines in Northern Virginia were sharing a single track to allow for repair work on a switch at Braddock Road.

Metro officials said the track work had been planned for several weeks and was described on the agency's Web site and in numerous announcements in stations since Thursday that suggested riders consider alternative transportation.

Jonathan Abel didn't get the message. The Alexandria resident said that after leaving the rally, he waited on a jammed platform at Pentagon City for about 45 minutes for a Yellow Line train toward Huntington.

"There wasn't an insurrection or a riot, but people were muttering," he said later. Told about Metro's track work, Abel said, "I'm a great believer in Metro, but they have certainly goofed in the past, and it seems absolutely incredible to me that they would pick this weekend to do their track work, when all of the free world is going to downtown D.C. for either the book fair or the protest."

Metro spokesman Steven Taubenkibel said the agency discussed at its board meeting Thursday whether to go ahead with the work, which was needed to replace the track switch outside the Braddock Road Station. A track switch allows trains to switch from one track to another. Officials decided that if they ran all six-car trains, instead of the usual four or six cars, they could handle the increase in riders.

As of 5 p.m. yesterday, Metrorail's ridership was 292,771, compared to 173,572 on a normal Saturday. The busiest stop was Smithsonian, with 33,819 people exiting the station.

"Here in the nation's capital, something is happening every weekend," Taubenkibel said. "We have to get this work done, and the best time is on a weekend. I'm sure people were frustrated by the delays, and we apologize for that." Taubenkibel said it was especially important to get the work done this weekend because Metro is on a compressed schedule, with other maintenance projects planned for various weekends in the fall.

The track work caused all Blue Line trains to share one track between Van Dorn Street and Pentagon City and all Yellow Line trains to single-track between Huntington and Pentagon City. Trains on both lines operated about every 38 minutes instead of the normal eight to 15 minutes.

Green Line trains were running on a single track between Greenbelt and West Hyattsville.

The work, which started Friday at 10 p.m., will continue until 5 a.m. tomorrow and then start again next weekend on the same schedule and continue until sometime next Sunday.

In New Jersey, Amtrak delays resulted from an accident during routine maintenance between Trenton and Newark at 4:50 a.m. Workers were operating a crane that was lifting a beam, and the beam fell onto electrical wires and shut all power to the area, said Vernae Graham, an Amtrak spokeswoman. She could not say how many passengers were on the 13 trains affected.

Finch said that at least 150 demonstrators were on her 6 a.m. train and that hundreds more were packed into Penn Station by 7 a.m.