But the worst lemons in the system may be the 34 escalators installed over the past seven years by Fujitec America at a cost of $65 million.

They include 12 escalators at the Herald Square subway station in Manhattan that have been plagued by a variety of mechanical problems, Mr. Joyce said. The motors in two of them vibrated so much that they were shut down for more than five months last year.

Fujitec began building escalators for the transit agency after its equipment was evaluated by Mr. Kenny’s office to make sure it was robust enough to hold up in the subway. Mr. Kenny said that it was not unusual for a company that was new to the New York subway to have problems with the first few escalators it installed. But he said that other companies had been more cooperative in resolving those problems. Mr. Joyce said that Fujitec was very slow to respond to repair requests, but that in recent months many longstanding problems had been fixed.

“Fujitec did not intentionally delay anything,” said Rick Lockridge, director of business development for Fujitec America. “But sometimes problems can occur in warranty that take some time to correct.”

In response to such widespread problems, the transit agency has changed the way it monitors elevator and escalator construction. For years, contractors installed machines with virtually no oversight from transit employees with expertise in elevators or escalators. In 2006, the agency established a special group of mechanics to conduct tests at key stages of construction.

The Disappearing Steps

On the morning of August 28, Lisa Chiou, 33, got off a subway train at the Bowling Green station in Lower Manhattan and got on an escalator that would carry her up and out of the station to Broadway. She placed her right foot (she was wearing flip-flops) on the lowest step, and as the escalator rose, she put her left foot on the step below it.

Except all of a sudden, there was no next step. There was a crash, and Ms. Chiou looked down to see that the escalator step that had been under her left foot had fallen out of the machine. It sheared off three more steps below it, and Ms. Chiou found herself slipping backward into the void.