It's called the Gaunless Bridge built in 1823 and it's claim to fame is that it was the first railway bridge constructed of iron. Designed by George Stephenson, part of the bridge can be found in York Railway Museum (the metal bit). The bridge is a short walk behind The Station pub in West Auckland, County Durham. The old illustration is to give an idea of how it looked. Time hasn't been to kind to it, but worth having a look.
Stone foundation of the Gaunless Bridge
Original Gaunless Bridge design