According to the final eis table 3-12, in 2030 there will be 2,815,800 trips per day by private car if rail is not built vs 2,767,600 trips per day by private car if it is built.  That's a whopping 1.8% less cars on the road - does that fit your definition of reducing traffic congestion significantly? All for a mere $10 billion, or a very reasonable $5.6 billion per percentage point improvement.  Awesome!