The Tay Bridge disaster occurred during a violent European windstorm on Sunday 28 December 1879, when the first Tay Rail Bridge collapsed as a North British Railway (NBR) passenger train on the Edinburgh to Aberdeen Line from Burntisland bound for its final destination of Dundee passed over it, killing all aboard.

[1] The bedrock lay much deeper than the trial borings had shown, and the bridge's designer, Sir Thomas Bouch, redesigned the span with fewer piers and correspondingly longer girders.

[5] The change in design increased cost and necessitated delay, intensified after two of the high girders fell when being lifted into place in February 1877.

A Board of Trade inspection was conducted over three days of good weather in February 1878; the bridge was passed for use by passenger traffic, subject to a 25 mph (40 km/h) speed limit.

The inspection report noted: When again visiting the spot I should wish, if possible, to have an opportunity of observing the effects of high wind when a train of carriages is running over the bridge.

Higher windspeeds were recorded over shorter intervals, but at the inquiry an expert witness warned of their unreliability and declined to estimate conditions at Dundee from readings taken elsewhere.

A court of inquiry (a judicial enquiry under Section 7 of the Regulation of Railways Act 1871 "into the causes of, and circumstances attending" the accident) was immediately set up: Henry Cadogan Rothery, Commissioner of Wrecks, presided; supported by Inspector of Railways William Yolland and William Henry Barlow, President of the Institution of Civil Engineers.

By 3 January 1880, they were taking evidence in Dundee; they then appointed Henry Law, a qualified civil engineer, to undertake detailed investigations.

Whilst awaiting his report they held further hearings in Dundee (26 February – 3 March); having got it they sat at Westminster (19 April – 8 May) to consider the engineering aspects of the collapse.

[23] By then the railway, the bridge's contractor and Bouch had separate legal representation, and the NBR had sought independent advice from James Brunlees and John Cochrane,[24] both engineers with extensive experience of major cast iron structures.

The terms of reference did not specify the underlying purpose of the inquiry – to prevent a repetition, to allocate blame, to apportion liability or culpability, or to establish what precisely had happened.

[33] Ex-provost Robertson had bought a season ticket between Dundee and Newport at the start of November, and became concerned about the speed of north-bound local trains through the high girders, which had been causing perceptible vibration, both vertical and lateral.

After complaining on three occasions to the stationmaster at Dundee, with no effect on train speed, after mid-December he had used his season ticket to travel south only, using the ferry for north-bound crossings.

[46] Whilst checking the pier foundations to see if the river bed was being scoured from around them, Noble had become aware that some diagonal tie bars were 'chattering',[note 10] and in October 1878 had begun remedying this.

Two "cotters" (metal wedges)[note 11] were then positioned to fill the rest of the slot overlap, and driven in hard to put the tie under tension.

Noble had found cracks in four column sections – one under the high girders, three to the north of them – which had then been bound with wrought iron hoops.

Moulds were damped with salt water,[50] cores were inadequately fastened, and moved, giving uneven column wall thickness.

[56] According to his predecessor, burning-on had only been carried out on temporary 'lifting columns', which were used to allow the girders to be lifted into place and were not part of the permanent bridge structure.

[74]Samples of the bridge materials, both cast and wrought iron, were tested by David Kirkaldy, as were a number of bolts, tiebars, and associated lugs.

[note 22] On the authority of Stewart they had assumed that the bridge was designed against a wind loading of twenty pounds per square foot (one kilopascal) 'with the usual margin of safety'.

[94] Brunlees had made no allowance for wind loading on the Solway viaduct because the spans were short and low – if he had had to, he would probably have designed against 30 psf (1.4 kPa) with a safety margin of 4–5 (by limiting strength of iron).

[97] Baker argued that the wind pressure on the high girders had been no more than 15 psf (0.72 kPa), from the absence of damage to vulnerable features on buildings in Dundee and the signal cabins at the south end of the bridge.

Baker's subsequent work on wind pressures at the Forth Rail Bridge site[98] showed meteorologists were overestimating,[99] but his 15 psf (0.72 kPa) might have over-interpreted the data.

[note 28] Law had numerous criticisms of the bridge design, some echoed by other engineers: Both Pole and Law had calculated the wind loading needed to overturn the bridge to be over 30 psf (1.4 kPa) (taking no credit for holding-down bolts fastening the windward columns to the pier masonry)[110] and concluded that a high wind should have overturned the bridge, rather than cause it to break up (Pole calculated the tension in the ties at 20 psf (0.96 kPa) windloading to be more than the 'usual margin of safety' value of 5 tons per square inch but still only half the failure tension.

[127] Dugald Drummond, responsible for NBR rolling stock, had examined the wheel flanges and found no 'bruises' – expected if they had smashed up chairs.

[130] The three members of the court failed to agree a report although there was much common ground:[131] Rothery added that, given the importance to the bridge design of the test borings showing shallow bedrock, Bouch should have taken greater pains, and looked at the cores himself.

[137] Yolland and Barlow concluded that the bridge had failed at the south end first; and made no explicit finding as to whether the train had hit the girders.

However, previous Section 7 inquiries had clearly felt themselves free to blame (Thorpe rail accident) or exculpate (Shipton-on-Cherwell train crash) identifiable individuals as they saw fit, and when Bouch's solicitor checked with Yolland and Barlow, they denied that they agreed with Rothery that "For these defects both in the design, the construction, and the maintenance, Sir Thomas Bouch is, in our opinion, mainly to blame.

[153][154] Condemning the structure, Colonel Yolland also stated his opinion that "piers constructed of cast-iron columns of the dimensions used in this viaduct should not in future be sanctioned by the Board of Trade.

224, a 4-4-0 designed by Thomas Wheatley and built at Cowlairs Works in 1871, was salvaged and repaired, remaining in service until 1919, nicknamed "The Diver"; many superstitious drivers were reluctant to take it over the new bridge.