Trowbridge Forest trail network expansion set to begin (3 Photos) A new link from Balsam Street will add another access point to the expanding trail system. Feb 22, 2021 2:23 PM By: Gary Rinne THUNDER BAY — Construction of a major expansion of Thunder Bay's mountain biking trail system is expected to begin in May and finish by October.. The city's Trowbridge Forest Recreational Trail Master Plan, developed in 2017, called for improvements to provide additional recreational opportunities for local residents and tourists, and potentially to attract national-level mountain bike competitions to Thunder Bay. The Trowbridge Forest mountain bike trail system – currently about 21 kilometres of single track – includes Centennial Park, Shuniah Mines, Trowbridge Falls Park and Kinsmen Park.