(This measure has not been amended since it was introduced. The summary has been expanded because action occurred on the measure.) Transit Security Grant Program Flexibility Act (Sec. 2) This bill amends the Implementing Recommendations of the 9/11 Commission Act of 2007 to permit the use of public transportation security assistance grant funds for backfill associated with security training. (Sec. 3) Funds provided pursuant to such a grant for a specified authorized use shall, with two exceptions, remain available for use by a grant recipient for at least 36 months. Any such funds used for security improvements for public transportation systems or security improvements for stations and other public transportation infrastructure, including those owned by state or local governments, shall remain available for at least 55 months. (Sec. 4) The Government Accountability Office shall review the public transportation security assistance grant program.