In the 1830s, two rival railways fought over how to reach Cornwall. The London and South Western Railway backed a central route, swinging north around Dartmoor from Exeter - easy to build, but serving very little in between. The Great Western Railway backed a coastal route through Plymouth and Devonport, harder to engineer but lined with naval traffic and the mineral wealth of St Austell. The Cornwall Railway, the GWR's local ally, applied to Parliament in 1845 with William Moorsom's idea of running trains across the Hamoaze on the Torpoint Ferry. Parliament said no. So Brunel took over as engineer and proposed something more ambitious: a bridge, higher upstream, at Saltash. The enabling Act passed on 3 August 1846. It took him another eight years to actually break ground.

Brunel's first instinct had been timber. The river is roughly 1,100 feet wide at Saltash; he sketched a double-track timber viaduct, central span of 255 feet, six approach spans, 80 feet of clearance above the water. The Admiralty, responsible for navigable waters, refused. He raised the clearance to 100 feet and reduced the spans to two of 300 and two of 200. The Admiralty refused again - the navy would tolerate only one pier in the navigable channel. Brunel went back to his drawing board, abandoned the timber double-track plan entirely, and came back with a single-track wrought-iron design featuring one improbable 850-foot span. The cost would have been £500,000 in 1846 money. Even Brunel blinked. He compromised again, settling on two main spans of 455 feet each, with a single midstream pier. This time the Admiralty said yes.

The trusses are shaped like lenses, and they work the way a lens works - by balancing opposing forces. The top chord of each truss is a heavy tubular wrought-iron arch in compression; under load it tries to lengthen. The bottom chord is a pair of suspension chains in tension; under load they try to shorten. Brunel sized the two so precisely that the changes cancel. The truss exerts no horizontal thrust on its piers, which mattered enormously, because the railway track curves on both approaches and any sideways push would have wrenched at the stonework. Between the chord and the chain, cross-bracing members and suspension standards hang down to carry the deck. It is one of the most elegant pieces of structural engineering in the nineteenth century, and it has worked, more or less unchanged, since 1857 when the first span was floated into place in front of a crowd of about 20,000.

Almost immediately, the bridge became a symbol. It was a labour of Hercules, declared an 1859 guidebook, but Mr Brunel has accomplished the feat. The Great Western's marketing department understood what they had. In its Cornish Riviera promotional copy, the writer SPB Mais described crossing the bridge as an almost magic means of transporting travellers from a county which is yet unmistakingly an English county to a Duchy which is in every respect un-English. The poet John Betjeman remembered the grey slate and back gardens of Plymouth giving way to the surprise of Saltash, the grey battleships moored below in the Tamar, and the steam ferry trying pathetically to compete with Brunel's mighty bridge. The structure has appeared on paintings, postcards, postage stamps, and on the reverse of the UK two-pound coin. ITV's regional news used to film against it. For 167 years it has been the moment you know you have arrived in Cornwall.

Brunel died on 15 September 1859, four months after the Prince's ribbon-cutting. His representative at the opening had been his chief assistant Robert Brereton. The directors of the Cornwall Railway placed the words I.K. BRUNEL ENGINEER 1859 in raised iron letters above the portals at each end of the bridge, and there they remain. The approach spans were rebuilt in the twentieth century and additional links between the suspension chains and decking added in 1969 to strengthen the structure. But the two great lenticular trusses, the impossible eyes of wrought iron 100 feet above the Tamar, are exactly the trusses Brunel designed. Trains still cross them every twenty minutes or so, carrying the Cornish Main Line in and out of the Duchy.