There was a dismantled railway line around there somewhere; maybe local people took some of it to reinforce the path in some way?

This bridge carried the now dismantled Hitchin to Leicester branch of the Midland Railway line. It was closed in 1962 as part of the infamous cuts imposed on the railways by Dr Beeching. It operated via Bedford and was opened in 1857.

This is from council archaeologist Keith Fitzpatrick-Matthews writing in a description of a circular walk starting at Wilbury Hill about an old bridge over the river that leads to the green footbridge over the railway (you get to it from near Ickleford church).

I interviewed Keith in my Icknield Way film which I will post about soon.