On the Mower County Board go to Maps there is a map of Austin 1940 this shows St Paul. As you are looking at the map St. Paul is one block to the left of Main Street.
My thinking is it was the railroad bridge that was over the tracks on the east side of town. This was a walking bridge. There was at the time the Railroad Depot, Hotel, ect. and many track to cross.
On the same map.
Find Water Street it runs east to west just south of the pond that has Red Cedar River on it. On the east end of town you will see where Water street ends at Railway St. This is where the walk over bridge was, connecting to Water St. on the other side of the tracks. Bridge is no longer there.