Stephenson's 1511 ft long iron tube was weakened by fire in 1970 and has been replaced by a series of steel arches. A second level has been added to carry the A5 across the estuary, diverting traffic from Telford's road bridge nearby.

Britannia Bridge was built to carry the Chester and Holyhead Railway over the Menai Strait in North Wales. The foundation stone was laid on 10th April 1846; the bridge took over three years to complete and Stephenson fitted the last rivet on 5th March 1850.

As engineer for the line, Stephenson was confronted with the difficulty of producing a bridge that could safely sustain locomotives travelling at high speed over an unusually wide span. The weather conditions were poor and the bridge had to maintain a level height so as not interfere with the navigation of the largest ships coming in from the Irish Sea.

Several early designs were rejected and Stephenson began to reconsider a roadbridge he had made in 1841, using trough-shaped iron girders. If the trough were closed across the top, would the girder then be self-supporting, even across significant spans? Rather than acting as a platform for the roadway, the tube might then carry the railway tracks through its middle.

He discussed the idea with the ironmaster, William Fairbairn, who experimented with the strength of this arrangement and the best shape for the iron tube. Professor Eaton Hodgkinson, a mathematician and writer on the strength of materials, was called upon to verify Fairburn's findings and produce formulae to describe and replicate them. The railway company funded this preparatory work to the tune of more than £6,000.

With Fairbairn's assurance, Stephenson eventually designed a bridge consisting of twin rectangular iron tubes running through four masonry support towers, without any further bracing or suspension.For the same railway line, Stephenson produced a similar design for a smaller bridge over the River Conwy. The construction of the iron tubes was borrowed from shipbuilding; they were made from riveted wrought iron plates.

Each of the Britannia Bridge tubes was made up of four spans connected end to end within the support towers. The two central spans are 470 ft each, with a 230 ft span at either side. To allow for expansion and contraction, Stephenson had the tubes fixed in the central towers but mounted them on rollerbeds in the side towers and abutments.

The side tubes were built first, in situ, on wooden platforms. Meanwhile, the two great central tubes, weighing 1800 tons apiece, were built on the Caenarfon shore and floated, one at a time, into position. They were then jacked into place using hydraulic pumps, with supports being built underneath at every few inches.The Conwy bridge tube had been successfully manoeuvred into position by the time the first Menai tube was ready. Nonetheless, Stephenson lost many nights' sleep over his method of construction.

Brunel stood with him to watch the first great tube float out on its pontoons in June 1849; he was later to use the same technique in building his bridge at Saltash.

Indeed, the success of Stephenson's bridges at Conwy and Menai changed the construction of iron bridges forever, and their tubular form can be considered the fore-runner of the modern box girder.