Bannock Pass, on the Idaho-Montana border and the Continental Divide, was by far the most substantial obstacle encountered by the G&P. The pass itself is at an elevation of , some higher than the center of Horse Prairie. Temporary trackage was built over the top of the pass during the G&P's construction, but a tunnel beneath the pass was clearly needed for the railway's permanent use. Since a tunnel to completely bypass the heavy grades of the pass was prohibitively expensive, the railroad compromised by boring a shorter, tunnel at the elevation. To bring the tracks to the elevation of the tunnel, the railroad line included a switchback on each side of the summit; trains ascending the pass would pull into the first switchback, run backwards through the summit tunnel, and then reverse direction again at the switchback on the opposite side.