To this day Northampton does not sit on the West Coast Main Line instead being on a branch loop between Hanslope Junction and Hillmorton Junction. The main line passes close to Blisworth over the bridge that is the subject of this cache and pictured below.
Stephenson's Railway Bridge, Blisworth, Northampton (CC-BY-SA)
There was a crude station opened here in 1838, but it didn't last long until it was replaced by a better one half a mile to the west (where there is another REALLY SideTracked cache GC2ZR2J). There are many more details on the Blisworth Archive website.