The 457-mile rail route from Washington to Boston is the busiest in the country, connecting four of the nation's 10 largest metropolitan areas and serving about a fifth of the U.S. economy. It's also the most bureaucratically tangled, with eight commuter lines, four freight railroads and Amtrak all sharing the same space. That's a sure-fire recipe for jurisdictional arguments over how to spend the $20 billion it takes to keep the corridor running.