Amends the National Trails System Act to direct the Secretary of the Interior to: (1) revise the feasibility and suitability studies for certain existing National Historic Trails for consideration of possible additions to such trails; and (2) study the feasibility and suitability of designating certain routes and cutoffs as components of the Oregon, Pony Express, California, and Mormon Pioneer National Historic Trails and as shared components of the California and Oregon National Historic Trails.