There are also stations with neither an elevator nor an escalator so riders have no choice but to use the stairs.

The task of keeping all of those escalators and elevators running falls to a team of about 250 maintenance workers. Mr. Byford, who has pledged to add 50 elevators to subway stations over the next five years, said he was also working to improve escalators.

“Recently, the escalator availability has dipped a bit because we have been doing more intensive maintenance, ” Mr. Byford said, meaning the escalators were actually down more often than usual for repairs.

But even new escalators can be undependable. The extra-long escalators at the new Hudson Yards station on the Far West Side of Manhattan have been plagued by problems.

One escalator at the station, where the platform is 125 feet below street level, has one of the worst availability rates in the entire system, at 78 percent. Transit officials did not provide a specific explanation, saying simply that they were investigating the situation.

The Hudson Yards escalators were installed by a contractor and are still under warranty, Mr. Byford said, so the contractor is responsible for maintaining them.

“We’re working with the installer to drive up their performance,” Mr. Byford said.

Safety is another factor. If an escalator’s safety systems are triggered, the escalator automatically shuts down and workers must inspect the equipment before restarting it.