With electric locomotives that top 200 miles per hour, travel time between New York and Boston would be slashed to 100 minutes — two hours quicker than current Acela service, the fastest train that Amtrak now runs. Construction would consume 20 years and require building the largest underwater tunnel in North America. The price tag: $105 billion. The president known as “Amtrak Joe,” who campaigned on a promise to invest up to $2 trillion on 21st-century infrastructure and “spark the second great railroad revolution,” has long been convinced of the virtues of rail. The project makes use of existing rights-of-way from the New Haven area to Hartford, but then requires yet another tunnel under Hartford, new tracks through environmentally sensitive areas in eastern Connecticut, and still another tunnel — the North-South Rail Link — under the “Big Dig” in Boston, to connect to modernized tracks heading to Boston’s northern suburbs, New Hampshire and Maine. Other branches of the network will require track modifications, straightening and electrification.