Transit to Trails Act of 2021 This bill directs the Department of Transportation (DOT) to establish the Transit to Trails Grant Program to award grants to certain governmental and private entities for projects that increase transportation access and mobility to public lands, inland and coastal waters, parkland, or monuments for critically underserved communities. For example, DOT must (1) prioritize approving certain projects, such as projects that use low-or zero-emission vehicles and those that provide free or discounted rates for low-income riders; (2) develop transportation planning procedures for projects conducted under the program that are consistent with metropolitan and statewide planning processes; and (3) ensure that all new transportation connectors and routes developed under the program are accessible to people with disabilities. The bill requires grant awards to be at least $25,000 and not more than $500,000. Further, the bill requires publicized schedules and routes for transportation systems that are developed after the receipt of a grant under the program.