The Dowd's Hill tunnel was built to broad gauge clearance specifications, pending a possible conversion. This was fortunate as standard gauge trains used it for a time during the changeover from narrow gauge.
Previously, the line ran over the top of Dowds Hill (which would have given it an extremely steep grade), the tunnel being built as part of a deviation to reduce gradients along the line.
Built in the 1880’s, it hardly seems to have been necessary, but it was the most convenient way at the time to allow for a road crossing, and in a sense was a glorified bridge.
There appears to have been a bit of competition between the various line builders of the different gauge lines who liked to try and outdo each other, so another tunnel on the narrow gauge line would have given their track a little more status as well.
You can just see the entrance to the tunnel as you look west from the cache.
Some time later, the tunnel was replaced by the adjacent cutting and the road made over the tracks on the Peterborough side (actually, the tunnel created by the overpass was made larger some time later again, but that’s another story).