More than 700 subway riders were evacuated yesterday morning after an electrical fire knocked out service on four major train lines and filled uptown tunnels with thin clouds of smoke, transit officials said.

At least three riders were reported to have been taken to hospitals suffering smoke inhalation.

At 11:02 a.m., as track workers were setting up for routine signal maintenance at the substation at 145th Street and St. Nicholas Avenue, an electrical fire broke out in a nearby transformer room, a spokeswoman for NYC Transit, Marisa Baldeo, said.

Though the precise cause of the fire is still under investigation, Ms. Baldeo said, the blaze knocked out power to the A,C,B, and D lines, bringing all four lines to a halt and ending much of Manhattan’s uptown subway service until 2:30 p.m.

“Today’s incident is not emblematic of any system wide breakdown or lack of maintenance. To the contrary, workers were in the substation following routine procedures to remove power, in preparation of capital rebuilding to the Concourse Line signal system,” a spokesman for the transit agency, Paul Fleuranges, said. “Obviously something went wrong. What, we don’t know.”

Train crews guided more than 100 people out from the maze of underground tunnels along the track beds, while passengers on a crowded northbound D train were shuttled out on a special evacuation train.

Five riders were treated for smoke-related injuries on the scene, a Fire Department chief, Joseph Callan, said. At least three, including a pregnant woman and an asthmatic, were taken to Harlem Hospital and Jacobi Medical Center. A transit worker was sent to Harlem Hospital to be treated for minor burns on his hands, Mr. Callan said.

While the evacuation occurred mostly without incident, riders were trapped in the stalled trains for at least a half-hour before rescue workers reached the first train, a member of the executive board of the Transit Workers Union, Dennis Boyd, said after he spoke with the train crews.

The emergency evacuation was further delayed when workers could not gain access to an emergency ladder used to hoist passengers from the track bed onto the platform, he said.

“You have emergency equipment that is kept under lock and key so that unauthorized personnel cannot access it, but neither can authorized personnel access it,” a union spokesman, Dave Katzman, said. “It just highlights the importance of looking at emergency procedures.”

Transit officials denied any knowledge of a problem with the evacuation ladders.