A railway bridge that stretches over the River Tamar (it gets its name from the station at the end furthest from London). It has one track only, and trains have to run quite slowly over it (presumably to stop the whole thing falling apart due to vibration as the bridge is made of iron-rust, anyone?), yet the view is awe inspiring, being 100ft above the water (with 80ft of leg beneath the water).

The bridge itself is a marvel of Industrial Revolution-era design, it's main feature being two large pipe-like trusses running along the top. While in the train car, you can see the very intricate design of the bridge up close-the long, thin, identical iron bars, stretching downwards at 45 degrees and bolted together at the bottom, supporting the two pipes (which altogether weigh 1000 tons).

The bridge was opened in 1859 and was designed by Isambard Kingdom Brunel (as was all of the Great Western Railway). Rail services (operated by Virgin Trains and First Great Western) travel over the bridge many times a day. I highly recommend you see for yourself, as the photos on the internet do it no justice.
Source: http://www.brunel.ac.uk/campus/history/saltash.html