Some members of the Thunderbolt ski Runners have been working with the DCR on single track near the Thunderbolt Ski Trail for over two years. The results are three miles of super swoopy fun trails up near the ski slope. The trail is an intermediate trail with switchbacks and climbing. There are a lot of other trails to ride in the Greylock Glen as well. you can easily put in a 10 mile ride and explore some great terrain. The ride down is super fun. Below is a map.