This is the last of the bridges on the River Lune that I visited earlier this year - continuing further downstream you would come to the Lune Millennium Bridge, but I have featured that previously.
The first bridge at the site of the Greyhound Bridge was a timber viaduct, built in 1849 to carry rail traffic to Morecambe. This was replaced in the 1860s with a more durable wrought iron structure, and then replaced again in 1911 with the riveted steel bridge that remains there today.
In the 1960s, the rail line was closed as part of the Beeching rail cuts, and in 1971-2 the bridge was converted to carry a highway and reopened. I can't readily think of other examples where this has happened. The diagram below shows how the bridge was altered:
The bridge's steel girders rest on cross-braced steel tubular caissons. The reinforced concrete highway deck was propped off the existing bridge crossframes with a series of steel stools, and cantilevers considerably beyond the original deck width. The works were designed by C. S. Allot and Son, later part of Allot and Lomax, and now absorbed (via Babtie) into the Jacobs empire.