I first learned of the remains of the Trolley Tunnel from a PBS show called "The Lost Twin Cities" back in the 1990's. This tunnel was part of the Twin Cities Rapid Transit Company. There is a big controversy of the demise of the well established trolley in it's day, and apparently, according to the PBS show, this tunnel and the tracks are the only visible sign that there were train tracks for the trolley system. The rest of the tracks have been removed, buried under concrete or covered with black top.
I had wanted to visit this tunnel for quite sometime, but never made time to do it. Finally I made it this summer. They had built the tunnel because the grade on the hill was too steep for the trolley cars, so they lessened the grade to make it up the steep hill towards the Cathedral.