The Appalachian Trail Centennial Act seeks to improve the preservation, maintenance, and management of national historic and national scenic trails by strengthening cooperative partnerships. It formally recognizes the critical role of volunteer organizations and other non-federal entities in the stewardship of these trails, distinguishing between inherently governmental administrative functions and delegable operational and management responsibilities. A central provision of the bill is the establishment of "Designated Operational Partners" (DOPs). The Appalachian Trail Conservancy is mandated to be designated for the Appalachian National Scenic Trail, and the Secretary concerned may designate other eligible non-profit entities for other covered trails. These DOPs are empowered to undertake various operational activities, including planning, construction, maintenance, and conservation, and can receive appropriated funds without competition. The Act outlines a process for DOPs to develop and submit proposed priority lists for land and resource protection, guiding federal land acquisition efforts. It also enables DOPs to request federal investigation and enforcement of property rights violations affecting trail resources, with provisions for potential litigation cost recovery. Furthermore, DOPs are granted the authority to accept or reject proposed comprehensive plans for their respective trails. To facilitate these partnerships, the bill clarifies that covered trails are eligible for various federal funding sources, including the Land and Water Conservation Fund. It encourages the Secretary concerned to enter into long-term cooperative agreements with volunteer organizations, delegating operational authority and sharing management responsibilities. The legislation also directs the Secretaries to establish a system for collecting and distributing permits and fees along the trails. Finally, the bill emphasizes the ongoing nature of trail planning and development, requiring periodic economic impact assessments on gateway communities and reports to Congress on trail development successes and challenges. It mandates that visitor capacity be determined for specific trail segments rather than the entire trail, ensuring tailored management approaches. Overall, the Act aims to foster a more collaborative and efficient framework for managing and sustaining the National Trails System.