On Thursday, an “unauthorized person on the tracks” in Brooklyn wreaked havoc on the A line. Power was turned off on the tracks in both directions for half an hour while the police searched for the man.

People who deliberately go onto the tracks can be charged with criminal trespass, a misdemeanor.

The incidents are also disturbing for subway workers and other riders who witness a gruesome death or injury. “It’s unbelievably traumatic for the train crew,” Mr. Byford said.

There have been a series of heartbreaking cases, including a woman who jumped onto the tracks with her 2-month-old baby this summer. But there have also been bizarre episodes — a woman strolling down outdoor tracks in Brooklyn last year, confounding other riders — and reports of frustrated riders escaping from stalled trains.

With a system that runs around the clock and no barrier standing between riders and the tracks, it is impossible to keep people from ending up where they should not be. But subway riders might not realize how perilous the tracks can be. They could be electrocuted by the third rail, a steel column that carries a wallop of 600 volts of electricity. It is also difficult to pull yourself back onto the platform once you are on the tracks.

“Don’t even think about it,” Mr. Byford said. “If you drop something, let us know. Do not go on the tracks under any circumstances.”

One solution that has been discussed for at least a decade is platform doors. London, for example, has glass walls lining some platforms to prevent people and objects from falling onto the tracks. But they are expensive, and officials at the Metropolitan Transportation Authority, which oversees the subway, have said that installing them across the system could cost more than $1 billion.

In June, Mr. Byford postponed a platform door trial at the Third Avenue station on the L line in Manhattan, instead choosing to spend the money on installing elevators at a different station. Mr. Byford said he understood the benefits of platform doors, but they were not a “cheap, quick or easy fix.” He instead wants to prioritize improving accessibility for wheelchair users.