But in case of fire, crime or a sick passenger  in fact, any other situation that could fairly be described as an emergency  the cord should be left alone. Stopping the train between stations will make it harder for help to arrive.

The explanation is on the agency’s Web site, albeit accessible only after several clicks.

“We think that it is clear,” said Charles Seaton, a spokesman for New York City Transit.

An unscientific survey of riders seemed to suggest otherwise. Of 20 straphangers interviewed last month at the 14th Street-Eighth Avenue station, about half said that they had no idea when the brake should or should not be used. Those who knew that the brake should not be pulled in most situations seemed at a loss to explain when exactly it would be appropriate.

“I can’t remember the last time I read that thing,” confessed Zev David Deans, 27, an artist who lives in Brooklyn, gesturing toward the emergency instructions poster on an L train.

Mr. Deans said that transit officials could do more to inform the public. “They could put it in big letters  ‘Pull in case of ...’  and then the few reasons why,” he said. “If it just says ‘emergency,’ you’re going to pull it for any reason.”