A push by Mr. Byford to improve the system has shown results. The on-time rate for trains jumped to nearly 80 percent last month — up from about 68 percent a year earlier. Officials have focused on making physical repairs to the aging infrastructure and speeding up the trains.

While the brake puller is unlikely to have affected the on-time rate, he has made more than a few New Yorkers late, and now joins an infamous list of people who have deliberately interfered with the city’s train operations, either because of an obsessive interest in trains or out of simple malevolence.

Perhaps the best known is Darius McCollum, known as the “train bandit,” who has been arrested 32 times for impersonating transit employees, stealing trains and buses and driving their routes. He has a passion for trains and his exploits were turned into a documentary, “Off The Rails.”

The offender’s apparent use of a key to enter the train cab raised questions over whether he knew someone at the transit agency. How the offender obtained the key is one factor being investigated by transit officials.

Brake cords are also located inside train cars so passengers can use them in an emergency.

Officials said that they were analyzing dozens of incidents since the start of the year that involve factors like brake activations or “surfer” sightings to determine the actual scope of the brake-pulling spree. They are also reviewing security footage and requested photos and videos from riders.

Each time, it was the same method, officials said. A man surfs on the back of the train, then gains access to an operating cabin. He goes inside, pulls the emergency brake and then escapes onto the track . The 2 and 4 lines were the most frequent targets.

The result, Mr. Byford said, is a “double whammy.” The person not only stops the train, but when he flees onto the tracks workers have to cut the power to look for him, delaying even more trains.