Commonwealth Mass Transit Fund. Allocates 3.5 percent of the Commonwealth Mass Transit Fund to commuter rail systems jointly operated by the Northern Virginia Transportation Commission (NVTC) and the Potomac and Rappahannock Transportation Commission and excludes such commuter rail systems from receiving allocations pursuant to other distributions of the Fund. The bill requires such commuter rail systems to submit reports to the Commonwealth Transportation Board. The bill limits allocations by the NVTC for distribution to the Washington Metropolitan Area Transit Authority (WMATA) to 50 percent of the total operating and capital assistance required to be provided by NVTC or other Virginia entities in the approved WMATA budget and establishes reporting requirements for WMATA. The bill codifies requirements for WMATA to adopt and submit certain planning documents first required pursuant to the eighth enactment of Chapter 854 and the eighth enactment of Chapter 856 of the Acts of Assembly of 2018 and repeals the original requirements.