After just two years on the job, New York’s subway chief, Andy Byford, has resigned, ready to move on from an impatient governor whose opinions on how to repair the ailing system often clashed with his own.

He is too polite to say as much, stressing in an interview with The Times that the start of the new year is just a good time to go. “It is absolutely my decision to leave,” he said.

Mr. Byford, a British mass transit expert willing to be the public face of a troubled train and bus network, brought hope to beleaguered riders. On-time train performance is now over 80 percent, up from 58 percent in January 2018, when he arrived. By the fall of last year, major subway delays on weekdays were down nearly 40 percent compared with a year earlier, according to data from the Metropolitan Transportation Authority, which runs the subway and bus system. Mr. Byford was taking common-sense steps like raising speed limits on the subways and increasing training for operators.

What had become a crisis for New Yorkers — a source of daily frustration if not anger as children arrived late to school and adults to work — had eased substantially, and even begun to show hopeful signs of turnaround. Together with the authority’s chairman, Patrick Foye, Mr. Byford, as president of New York City Transit, was overseeing a long-overdue transformation of an aging subway system in dire need of investment.