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This is a steel and masonry viaduct carrying the railway between Elgin and Keith over the River Spey near Orton, north of Forres.
For location map and photos, see Railscot entry.
Built in 1858 for the Inverness and Perth Junction Railway. It was designed by Joseph Mitchell, and the original wrought ironwork was produced by the William Fairbairn and Sons of Manchester. When the wooden staging was removed, the girders deflected excessively, and subsequently had to be reinforced.
R. Byrom states that the problem was one of the factors which led Messrs Fairbairn to embark on extensive load testing of riveted bridge structures. The tests, which included fatigue testing, were carried out in their Manchester works, with the Treasury granting £150 towards the costs, for 'experiments to ascertain the durability, and the measure of strength to be allowed, of wrought-iron bridges, subjected to changes, shocks and vibrations of a continued and variable load.' 
The wrought iron tubular (box) girders were replaced by steel trusses in 1906. These were about twice the height of the box girders, and were erected between them (the bridge was built for double track, but the replacement span was narrowed to single track).
Photographs taken during the 1906 reconstruction show that the box girders box are topped by box section structures, having two cells approximately 18" square, similar to those at Torksey. It has been suggested that these may have been added as part of the strengthening work, noting that the 1858 article below refers to the top being closed by a plate. However, it would be surprising if the top chords had not been made as box section from the outset, given that the much shorter girders of the earlier Torksey Viaduct were of box section.
1858 'IRON BRIDGE OVER THE SPEY. ..... The river is crossed by one span — a bridge of iron. Yet, contrary to what the uninitiated might be apt to suppose, the bridge is not one solid mass. If the iron were solid, the weight would be so great that it would crush the masonry of the piers to atoms. The question is, how arc we to get strength without weight ? The difficulty has been solved by Mr Fairbairn, the eminent Manchester engineer, the maker of the bridge. If you take two sheets of tin and place them perpendicularly, they will bear a much greater weight than if held horizontally. It is this principle that is used in the bridge. The plates of iron, 16 feet high, 5-8th of an inch thick are raised perpendicularly, about three feet apart from each other. These two plates are further covered on the top by a plate of similar thickness, which is fastened to the side plates by rivets. A similar plate covers the bottom. Strong bolts, firmly riveted, are passed between the two side plates, in two or three rows along the whole length. You have thus a hollow beam of iron 16 feet high and rather more than three feet thick. In such a beam it has been proved you have all the strength which a beam of the same dimensions would have, while you have got rid of the terrible weight. By adding plates you can make your beam of any length. Thus the beam which is laid upon the stone piers placed on either side of the Spey is 245 feet long; seven and a half feet rest on the piers on each side, making the exact free span 230 feet — a great width! The reader must understand — and the traveller will by and bye (not now, for the above sketch shows the bridge as it is to be) see that there are, of course there must be, two beams — one on either side; the rails are laid between them, and are supported on a plate of iron rivetted to the bottom of the two beams. The roadway between the two beams is 24 feet, and will admit of the laying of two sets rails. It may be well to add, giving an idea of the height the bridge, that the distance from the bed of the to the level of the rails ... At present, however, the trains cross the roadway on temporary scaffolding. The trains all stop when they come to the bridge, and all travellers who are timorous are conveyed in an omnibus across the suspension bridge, which is only a few yards above the railway viaduct. The suspension bridge was built about the year 1830 ; and it is a coincidence perhaps worthy of notice that it was erected, we believe, under the care of Mr Mitchell, younger brother of Mr Mitchell's, the talented engineer of the Inverness and Aberdeen Railway, under whose superintendence the magnificent railway bridge has been built. The younger Mr Mitchell was at one time assistant to Captain Brown, the inventor of the suspension principle. At the spot where the suspension bridge now stands, there was one time a wooden bridge, which is believed to have been first thrown across the Romans. ...'
1860 'To the Editor of the Elgin & Morayshire Courier.
TUBULAR BRIDGE OVER THE SPEY.
Sir, — In a leading article ia the Engineer of 9th inst., on "Railway Bridges," the following passage occurs. The Engineer is discussing the question of the relative merits of plate-iron and lattice-girders, in consequence of rumours of an unsatisfactory nature, respecting the security of the Boyne Viaduct, on the Dublin and Belfast Junction Railway, and other similar structures. The article goes on to say :— " Whilst, however, there has been no occasion, thus far, to doubt the security of the girders, [that is, girders of the lattice description] this is much more than can be said of some other bridges constructed upon the "tubular," or plate-girder principle. The alleged weakness of the Torksey Bridge, is stiil fresh in the minds of Engineers, and the Spey Viaduct has more recently furnished the profession with another illustration of the possible insecurity of structures of that description. This viaduct, comprising a pair of box girders of 230 feet clear span, deflected 3 in. by its own weight, and, after a permanent set of 1 in. more, deflected 3 in. further, under rolling load of 2 tons per lineal foot — a total deflection of 7 inches. Captain Tyler was compelled to report to the Board of Trade, that the bridge could not be opened without danger to the public using it, and it was only upon formal undertaking to strengthen it, that the Aberdeen and Inverness Junction Railway Company were allowed to open it all." In a matter of this nature which so largely involves the safely of life and property, it might be proper, seeing the subjct has got ventilated in public print, that some satisfactory assurance were given by those able to give it, of the means adopted for the security of the structure question.
I am. Sir, &c., An Enquirer.'