Welcome to the Iron Horse Trail. This Trail was built on the Southern Pacific Railroad San Ramon Branch right-of-way. The right-of-way was abandoned in 1977 but it took many years of grassroots efforts to establish the trail. The Iron Horse Trail, when completed will span from Suisun Bay to Livermore.
There are lots of other abandoned RR right-of ways that have been turned into trails. The Rails-to-Trails Conservancy is one such group that helps organize such efforts. There are other rails-to trails in the East Bay. They include the Ohlone Greenway in Berkeley/ Albany, the Richmond Greenway in Richmond, and the Lafayette/Moraga Regional Trail in Lamorinda.
To qualify for this challenge you must find 50 caches that have been placed on a trail that was built on an abandoned railroad right-of-way. You can use caches found on the trails I mentioned or on any other rail-to-trail in the world. There are no mean icon or found-after-date restrictions. Just come out and grab this cache and show proof of your finds.