From their event page: 

Join Fairmount Park Conservancy for a guided 4-mile hike to learn about the Trolley Trail’s fascinating past and present. The Fairmount Park Trolley operated in East and West Park from 1896-1946, providing a reliable and enjoyable means of accessing park amenities. Explore the physical remains of what is now a hidden trolley network including massive stone tunnels and bridge abutments along this loop hike.

More about the Trolley Trail:Fairmount Park Conservancy, along with partners at Philadelphia Parks & Recreation and the Belmont Plateau Trails Alliance, have been working to develop the Trolley Trail — a scenic multi-purpose trail that loosely follows the path of the former Fairmount Park Trolley. New gateways, trail signage, and bike racks were recently added and a ribbon cutting was held in November 2023. Read more here.