The term TRAIN THAT SERVES THREE UNION STATIONS is referring to the Acela Express, a high-speed train operated by Amtrak that runs along the Northeast Corridor between Washington, D.C., New York City, and Boston. The Acela Express is the only high-speed train in the United States and is capable of reaching speeds of up to 150 mph. It is the fastest way to travel between the three cities, with a journey time of just over three hours.