City Council Speaker Corey Johnson said he was disappointed to learn Andy Byford, the MTA's Chief Executive Officer of the New York City Transit Authority, is resigning.

"It may be one of the toughest jobs in the world running the subways and buses and he did an outstanding job," Johnson said. "It's a shame, it's unacceptable. I don't think he wanted to leave. I think Andy is someone who has really turned the system around in a short period of time."

Byford submitted his resignation on Thursday. Speaking before an MTA board meeting, he highlighted the progress the transit authority made during his time at the helm and thanked New Yorkers for welcoming him aboard.

That progress is on the mind of local lawmakers. They credit Byford for making significant strides, and wonder if it's all now at risk.

"I thought Andy was doing a great job. You have the MTA in the throes of a complete crisis of confidence, and you had Andy, who was acknowledging that and taking public steps to try to build back the public's trust," said Councilman Justin Brannan, who represents parts of Brooklyn.

The news came as a surprise for others, including Councilman Keith Powers, who represents parts of Manhattan and whose district includes the L Train project.

"It was a surprise. Absolutely terrible news for New York City residents because he was really the person spearheading the transformation of the MTA," Powers said.

In his resignation letter, Byford pointed to his job description being diminished as part of a reorganization of the MTA.

Councilman Joe Borelli, a Republican from Staten Island, blamed Cuomo for what some have described as power struggle.

"Gov. Cuomo really is fundamentally responsible for the decisions of the MTA, and I guess he didn’t like that. So when your train starts to break and progress is rolled back, you can blame Gov. Andrew Cuomo," Borelli said. "Unfortunately, we lost the good guy and we're stuck with Andrew."

The last day of work for Byford, a British transplant who became known for riding the system while wearing a name tag, is February 21. For now, officials are faced with the challenge of finding a leader for a system serving 7 million New Yorkers each day.