Conceived in the days following the Sept. 11, 2001, attacks, the September 11th National Memorial Trail has been in the works for nearly 18 years. Over that time, the 1,300-mile, multistate trail has come together through a mixture of roadways, existing trails, and new trails that link the sites of the National September 11 Memorial & Museum in New York, the Pentagon Memorial in Arlington, Va., and the Flight 93 National Memorial near Shanksville, Pa.