It’s a bad idea to make a bridge out of cast iron – it’s brittle and doesn’t handle tension well – but the very first major bridge of this type opened to traffic in 1781 and still stands today.
The Iron Bridge (creative name, by the way) spans the River Severn in Shropshire, England. It’s actually quite beautiful, stretching over the river in a half-circle arch. The Iron Bridge survived a 1795 flood that destroyed all the other bridges over this part of the Severn, bits falling off in 1902-3, and a proposal to demolish it in 1956.
The bridge was only closed to traffic in the 1930s; today you can walk across it and admire this early artefact of the Industrial Revolution.