Light Rail Transit Act This bill directs the Federal Transit Administration (FTA) to establish a grant program to provide grants to state, local, and tribal governments for the design and implementation of light rail projects, including for the redesign, retrofit, renovation, update, and repair of existing light rail systems. As condition of receiving a grant, eligible entities must certify that fares for riding the light rail transit system shall not increase solely due to improvements carried out with grant funds and for any reason for at least one year after completion of a project funded by the bill. In carrying out the program, the FTA must prioritize projects located in economically disadvantaged communities.