Southwestern Transit Corridor Planning and Fuel Use Reduction Act - Directs the Secretary of Transportation to make a grant to: (1) the New Mexico Mid Region Council of Governments to study the feasibility of extending the New Mexico rail runner express commuter rail service south from Belen, New Mexico, to El Paso, Texas, or to intermediate points; and (2) the city of Las Cruces, New Mexico, and the city of El Paso, Texas, to study the feasibility for a public transit system, including the option of commuter rail, that would connect Las Cruces and El Paso.