The future of rail travel along between Boston, New York, and Washington, D.C. could be underwater, if a proposal from the University of Pennsylvania School of Design is adopted by Amtrak.

The idea, the Hartford Courant reports, is to build an 18-mile tunnel under the Long Island Sound, from Bridgeport to Terryville, on Long Island.

Amtrak is working to bring high-speed rail (HSR) to the Northeast Corridor (NEC), which covers 457 miles between Boston and Washington. It carries 2,000 Amtrak and commuter rail trains each day. An HSR network would require two new sets of tracks to accommodate trains that run at more than 200 mph. Amtrak hopes to provide service from New York to Washington in just 94 minutes by 2040.

PennDesign calls for 87 miles of tunnels among more than 800 miles of new track, which serve, it says, to straighten curves and avoid irregular topography, vital for high-speed travel. The route would cut south from New Haven, under the Long Island Sound. It would run through the center of Long Island, into Manhattan.

The HSR line Amtrak wants to build, unveiled in 2010 and updated in July 2012, uses a conventional route, from central Connecticut, into Westchester and the South Bronx, then Manhattan. It would cost $117 billion.

PennDesign predicts the 18-mile tunnel would cost $20 billion, and the full network $100 billion. The plan includes ideas for public-private financing, while Amtrak has not outlined specifics for how it would raise funds for its HSR system.

PennDesign's proposed HSR route. PennDesign