The Appalachian Trail is a mentioned only hiking trail.
The trail is mentioned by two individuals. Miguel Caldera, a pre-War programmer for the Vault-Tec Corporation, longed to be free from the shackles of modern life. His dreams of never having to work, being able to hike the Appalachian Trail every day with his friends, are what kept him going at his job.
After the bombs fell, Surge's father died of heart failure. She decided to start walking the Appalachian Trail for no particular reason. At some point along her journey, she spotted Lev in a firefight with a group of super mutants through her scope, and decided that she should join up with him.