Eric Jaffe, writing for the Atlantic Cities:
Very few U.S. travelers go through an airport by intercity train as a result. One recent study found that Amtrak accounted for just 3 percent of all airport access at Newark, 2 percent at Baltimore, and less than 1 percent at Burbank. Newark is the only airport with an agreement between Amtrak and an air carrier (United) to let passengers reserve seats on both modes at the same time.
Contrast those figures with rail-air connectivity measures in Europe. There, intercity rail accounts for 20 to 25 percent of travel access at some airports. The European Commission has made air-rail integration — or air-high-speed rail integration, to be more precise — a major priority, adopting a resolution to connect all 37 core airports to intercity rail by 2050.