His departure has been the subject of some speculation for weeks, as the restructuring of the M.T.A. that accompanied its bailout—finally agreed upon Wednesday night—called for the consolidation of the executive director and chairman positions. 

Mr. Sander's resignation does not appear to be of his own volition, given that he said just Wednesday that he had no plans to leave. If so, the Paterson administration is pushing out a man who was widely viewed by both transit advocates and the construction industry as a highly competent transit professional—a policy wonk whose political skills were apparently not quite good enough.

The M.T.A.'s press release did not name a successor, but the name most widely discussed among transit advocates is Marc Shaw, a Paterson adviser who was the agency’s executive director during the Pataki administration.

In a statement, Mr. Sander hailed a long list of accomplishments during his tenure and added:

“There is more work to be done, but I leave confident knowing the M.T.A. is headed in the right direction. I am grateful to Governor Paterson and Governor Spitzer for this wonderful opportunity. I wish Governor Paterson the best of luck in choosing a successor who will build on the progress the M.T.A. has made over the past two and a half years."