This chain suspension bridge was opened in 1830 to carry Stockton and Darlington Railway trains to Port Darlington, (now Middlesbrough). It was the first suspension bridge built for railway traffic, and proved totally unsuitable due to deck flexure. It was necessary to prop the bridge at mid span and to severely limit the loading.
The bridge was replaced by a trussed cast iron girder bridge, opened in 1844, but this had to be propped following the failure of the Dee Bridge, which had a similar type of girders, in 1847.
For more information, see Wikipedia entry.