Long a punch line for harried Northeast travelers, Amtrak has come to dominate commercial travel in the corridor connecting Washington, New York and Boston, and this summer its trains are packed.

A decade ago, Delta and US Airways shuttles were the preferred mode of travel between the cities. But high fares, slow airport security and frequent flight delays — along with Amtrak’s high-speed Acela trains, online ticketing and workstation amenities — have eaten away at the airlines’ share of passengers.