The original Tay Bridge was designed by noted railway engineer Sir Thomas Bouch, who received a knighthood following the bridge's completion. It was a lattice-girder design, combining cast and wrought iron. The design was well known, having been used first by Kennard in the Crumlin Viaduct in South Wales in 1858, following the innovative use of cast iron in The Crystal Palace. However, the Crystal Palace was not as heavily loaded as a railway bridge. A previous cast iron design, the Dee bridge which collapsed in 1847, failed due to poor use of cast-iron girders. Later, Gustave Eiffel used a similar design to create several large viaducts in the Massif Central (1867).

Proposals for constructing a bridge across the River Tay date back to at least 1854. Its importance lay in speeding access to and from Aberdeen. The North British Railway (Tay Bridge) Act received the Royal Assent on 15 July 1870 and the foundation stone was laid on 22 July 1871. As the bridge extended out into the river, it shortly became clear that the original survey of the estuary had not been competent. The bedrock, at a shallow depth near the banks, was found to descend deeper and deeper, until it was too deep to act as a foundation for the bridge piers. Bouch had to redesign the piers, and to set them very deep in the estuary bed to compensate for having no support underneath. He also reduced the number of piers by making the spans of the superstructure girders longer than before. The first locomotive crossed the bridge on 22 September 1877, and upon its completion in early 1878 the Tay Bridge was the longest in the world. The bridge was opened on 1 June 1878.

On 28 December 1879, the first bridge collapsed during a violent storm, while a train was crossing it. Seventy-five people were killed, including Sir Thomas' son-in-law. The inquest found that Bouch had not included wind pressure calculations in his design of the bridge - in fact this was not practised at the time. The section in the middle of the bridge, presented high wind resistance where the rail ran inside high girders (through trusses), rather than on top of lower ones (deck trusses). This was done to allow a sea lane below high enough for the masts of ships. As a result the bridge was potentially top heavy and very vulnerable to high winds. Furthermore, neither Bouch nor the contractor appeared to have regularly visited the on-site foundry where iron from the previous half-built bridge was recycled. The cylindrical cast iron columns supporting the 13 longest spans of the bridge, each 245 ft (75 m) long, were of poor quality. Many had been cast horizontally, with the result that the walls were not of even thickness, and there was some evidence that imperfect castings were disguised from the (very inadequate) quality control inspections. In particular, some of the lugs used as attachment points for the wrought iron bracing bars had been "burnt on" rather than cast with the columns. All of the high girders section fell during the accident, and analysis of the archives has shown that the design of cast iron columns with integral cast iron and therefore inflexible lugs holding the tie bars was a critical mistake. There were other problems with the bridge most of which had arisen from cost-cutting (the North British Railway which paid for the bridge was over-stretching itself) such as poor smelting for the iron work and the re-use of girder sections which had been dropped on the sea-bed during construction. The enquiry concluded that the bridge was " badly designed, badly built and badly maintained".

A new double-track bridge was designed by William Henry Barlow and built by William Arrol & Co. 60 ft (18 m) slightly upstream of, and parallel to, the original bridge. The bridge proposal was formally incorporated in July 1881 and the foundation stone laid on 6 July 1883. William Barlow led the design of the replacement bridge (1882-87) with his son Crawford Barlow. They used massive monocoque piers to support a double railway track. They were made from wrought iron and steel. The old brick and masonry piers from the first bridge built by Thomas Bouch were retained as breakwaters for the new piers upstream. They can still be seen today as a forlorn reminder of the tragedy of 1879.

Construction involved 25,000 tons of iron and steel, 70,000 tons of concrete, ten million bricks (weighing 37,500 tons) and three million rivets. The second bridge opened on 13 July 1887 and remains in use. It is the longest railway bridge in Britain. When built it was the longest in the world.

Photo: TheCreator, Public domain, via Wikimedia Commons