There have been a number of references to the Avon bridge in recent publications and unfortunatly some mis-information has surfaced. I have corrected this situation elsewhere on this site but would now like help on dating the first major strengthening of the bridge.
The E & W J R could have had little thought when they built the bridge as to what loads it would carry during its lifetime. In the event two major acts of strengthening were carried out post WW2 firstly by BR in the late 1950's as part of the upgrade to carry both steel billet traffic and iron ore from 1960 onwards. The bridge received further strengthening work by the highway authorities to produce the situation we see today in its continuing role as a road bridge.
Examination of photographs shows that much earlier works were carried out because the round cast steel plates that are still in place today and presumably are connected through the brickwork to "tie in" the walls do not feature in photos until the early 1930's. I have attached a 1910 photo showing the bridge in original form without the round plates. My guess is that such work (and cost) was almost certainly LMS not SMJ and possibly connected with the improvements to the line in the 1920's. Can anyone confirm this theory and are there any previously unseen photos of the bridge that are dated?
One thing is certain. The early promoters of the line and the original engineers could not have dreamt about the axle weights carried in its later days!!
I don't know too much about the Western end of the line, but certainly the LMS carried out considerable work on the Olney Branch during the late 1920/early 30s. Some twenty years ago, when I was running night classes in local history at Roade, one of my students was an elderly gent from Horton (more or less the next village to Ravenstone where the junction with the Midland was). He told us that he could recall the whole line being relaid when he was a boy between Ravenstone and Salcey Forest. The line through Stoke Bruerne was similarly relaid with ex-L&NWR material, because I recovered a chair from the section above Blisworth Canal Tunnel in 1966, soon after the track was finally taken up. The chair had the cast mark of Crewe, 1909 and I used it as a doorstop at work for many years until sombody pinched it!
On this basis I would think it most likely that the LMS relaid and rebuilt such vulnerable structures as the Avon bridge, especially since the Broom to Byfield section carried heavy freight locomotives after passenger services were withdrawn in 1952. Any evidence needs to be recorded, but I dare say there may be a reference somewhere among the LMS papers at Kew.