Valerie Brown stepped down as chairwoman of the Sonoma-Marin Area Rail Transit district Wednesday, after leading the rail agency through some of its most tumultuous times.

Brown, who has also retired from the Sonoma County Board of Supervisors, had been on the board since 2009.

She took over as chair of the agency when Marin County Supervisor Charles McGlashan died suddenly in March 2011.

Brown worked to unite the directors representing Sonoma and Marin counties, get the agency past an effort to repeal the underlying sales tax, through the sales of construction bonds and into the work of building the commute rail system.

"Today is the end of an era," said Marin County Supervisor Judy Arnold, SMART's vice chairwoman. Since Brown took over "it has been a well-oiled machine that is bringing trains to town."