Polly Trottenberg, who has served as one of Mayor Bill de Blasio’s four appointees on the MTA board, resigned on Wednesday.

“It has been an honor to serve with so many dedicated Board colleagues over these past five years. My resignation will be effective upon the nomination and Senate confirmation of a successor who can ensure that New York City maintains a full four-member delegation on the Board,” said Trottenberg, the current New York City Commissioner for Transportation.

Trottenberg did not reply to email inquiries about why she was resigning now.

To replace her, de Blasio has nominated Bob Linn, a former Commissioner of the Office of Labor Relations. Another one of the mayor’s appointees, Carl Weisbrod, stepped down in March. The mayor nominated Dan Zarrilli to fill his seat.

It’s now up to Governor Andrew Cuomo to approve and submit these two nominations to the Senate for a final approval. There are roughly two weeks left in this year’s legislative session.

In Trottenberg’s case, she’ll continue to serve on the board until her replacement is confirmed.

An outspoken board member, she was one of the first members to vote against continuing the Enhanced Station Initiatives, the $1 billion Cuomo-driven program that beautified stations without adding elevators.

"We're having a pretty intense debate about every penny. I think we all agree, let's spend it in the way that gets the most bang for the buck," Trottenberg said at the time.

Trottenberg’s frustration seemed to be growing month over month. During an emergency L train meeting she pointed out that the MTA had effectively cancelled the L train plans without consulting or getting approval from the board. “Do we have any actual role here?” she asked.

Her departure comes as the MTA is finalizing it’s next five-year capital plan, one of its most transformative, as the agency grapples with how to implement Andy Byford’s $40 billion Fast Forward plan. Also, it dovetails with increased attention on the agency by Cuomo, who told WNYC on Tuesday that he would not give more than $30 billion to the MTA’s next capital plan.

Cuomo is apparently planning to stock the MTA with more loyalists. And this month he replaced the former MTA Inspector General with a lawyer from his office .

Cuomo’s office did not return requests for comment on whether or when he’ll approve the mayor’s new nominees for the board.

Update: In a statement, MTA Chairman Pat Foye said, "Polly has been an terrific board member and colleague, and we’ll miss her hard work, unique insight and intelligence. I join all New Yorkers in offering her a heartfelt ‘thank you’ for her years of service on the MTA board. I look forward to continuing to work with her while she remains on the board, and seeking her advice and counsel after she leaves."

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