Rainey, 51, was police chief in Fairfield from April 2007 to September 2009 and most recently had been the commander of San Antonio's airport police division. He assumed his duties as BART's police chief on June 16.
He was sworn in today in front of his family and many community leaders.
Former San Francisco Mayor Willie Brown, who presided over the ceremony, described Rainey as "a young man I've had the great pleasure of knowing for many years."
Rainey takes over a Police Department that has been under scrutiny since former BART officer Johannes Mehserle killed unarmed train rider Oscar Grant III at the Fruitvale station in Oakland on Jan. 1, 2009.
Closing arguments are being presented today in Los Angeles County Superior Court in Mehserle's trial on a murder charge stemming from the shooting.
Oakland community leaders and police are bracing for the possibility of civil unrest when the verdict is announced, and Rainey told reporters after today's ceremony that "we're doing the things that other agencies are doing" to prepare for protests.
A native of Chicago, Rainey worked for the Ventura County Sheriff's Office for 23 years before leaving as a captain in May 2002.
He was the superintendent of patrol operations in Dayton, Ohio, which is the department's No. 3 job, from June 2002 to April 2004 and was the patrol operations captain in Whittier, which is that department's No. 2 job, from April 2004 to April 2007.
Rainey succeeds former BART Police Chief Gary Gee, who announced his retirement last August. Gee went on medical leave in September but returned to his post in December for a few weeks. His last day on the job was Dec. 30.
Former Berkeley Police Chief Daschel Butler served as interim police chief after Gee left.
BART officials said last December that they hoped to have a new police chief on the job by April 26 but the selection process took longer than expected.
BART director Lynette Sweet, who's a member of the agency's police review committee that was formed after Grant was killed, told Rainey at the ceremony, "What's so great is the calming effect you have on just about everyone. You are the right person for this police department at this time."