It is one of the longest units of the National Park System in the country. "It," of course, is the Oregon National Historic Trail, which stretches more than 2,100 miles from Missouri to Oregon. It’s been estimated that between 1840 and 1860 some 300,000-400,000 men, women, and children embarked on the four-month-long trip to head to the West Coast.  A new book, “Exploring The Oregon Trail, America's Historic Road Trip,” can help you explore sections of the trail, or the entire trail if you have the time. It was written by Kay and David Scott, longtime national park travelers and park lodging experts.