California Code, Streets and Highways Code - SHC § 146

Any public agency having responsibility for the planning and development within a region of this state of public transportation systems may, with the approval of the commission, use the airspace over or under any existing state freeway in that region, or any portion other than the travel roadway of the right-of-way of such freeway, as a route for a public transportation system, such as a railway, monorail, tracked air cushion vehicle system or other such system which, in the discretion of the agency and the department, is feasible from an engineering standpoint, in conformity with established safety design standards, and is consistent with good ecological and environmental planning.  The development and construction of such facilities shall be financed out of any available federal, state, and local funds.