order to accommodate new housing development and the new Warriors stadium that was in planning at the time. With the 2015 alternative Mission Bay alignment, trains would potentially run underground along Third Street. The trains would then bypass the existing terminus at Fourth and King, enabling the redevelopment of the Caltrain station and potential addition of infill housing. Another suggested revision included demolishing part of Interstate 280 to facilitate further development. Other proposed DTX routings included a loop so that trains may leave TTC as other trains are pulling in. The study for these revisions, coordinated by the San Francisco Planning department, was called the Railyard Alternatives and I-280 Boulevard Feasibility (RAB) Study and was initially projected to complete in eight months. One of the goals of RAB was to propose grade separations for the surface rail segments, as the planned frequency of trains with the completion of CalMod and CAHSR meant that at-grade crossings at Mission Bay Drive and 16th Street could be blocked more than of the time during peak commute hours. After five years, the results of the RAB study were released in May 2018. The RAB study considered three possible alternatives that would extend or replace the environmentally cleared DTX tunnel alignment: Future with Surface Rail: DTX plus two grade separations created by trenching and depressing Mission Bay Dr & 16th St Pennsylvania Ave Alignment: A tunneled extension of DTX under Pennsylvania Avenue Mission Bay Alignment: An alternative tunneled extension of DTX under Third Street in Mission Bay The study ultimately recommended the second alternative. Alternative 1 would create underpasses four to five stories deep and more than long, reducing pedestrian mobility and leaving the roads vulnerable to sea level rise. In addition, assuming the surface railyard at 4th and King is retained, trains would need a turnback track and a new portal near 6th and Townsend to avoid cumbersome maneuvers to access the portal at 7th and Berry. Both Alternatives 2 and 3 would remove the surface railyard and relocate it, presumably to a point further south in San Francisco. Alternative 3 would use potentially the largest tunnel boring machine in the United States for a single bore with stacked train tracks, and discards much of the route for DTX that had already received clearance; in addition, the effects on existing structures (such as the Lefty O'Doul Bridge and Oracle Park) would require further study,