By the middle of the 19th century the rush to exploit the vast coal reserves of the South Wales valleys was well underway. The development of the railway network in the area played a vital part in this process. But the deep valleys and steep sided hillsides proved a formidable challenge to the railway engineers of the time.

By 1847 it became obvious that a new double track railway line of standard gauge was needed to link the NAH (Newport, Abergavenny & Hereford) in the east, with the Taff Vale Railway in the west. Parliament authorized the NAH to build this line, which was called the TVE (Taff Vale Extension). Unlike the other railways in the area which all followed the valleys down to the ports, this one running east/west had to cut across the valleys, which entailed a number of tunnels and bridges.

At the crossing of the Ebbw and the smaller Kendon valley at Crumlin, it was decided that a stone bridge would be impracticable owing to the strong lateral forces of the wind in such a narrow valley. But the more likely reason for not opting for a masonry structure was because of the enormous cost involved. In August 1852 the Board of the N.A.H. invited tenders for the construction of a wrought iron bridge and by October they had received two. One was a lattice structure submitted by Doyne, who had previously designed a bridge over the river Taff at Treforest and the other by Kennard described as a Warren Truss Bridge. Charles Liddell the chief engineer with the N.A.H. preferred Thomas Kennard's submission and this was accepted by the company and the contract was signed in December that year, with a stipulated completion date of October 1st 1854.

Kennard's plan incorporated his own-patented modification of the Warren Truss (a form of bridge span invented by James Warren). There would be ten Trusses in total, each being 45m.(150 ft.) in length and supported by eight Piers. Six Piers and seven Trusses over the Ebbw valley, two Piers and three Trusses over the Kendon valley, with the hillside between forming a further natural support. Kennard himself designed the Piers, which were formed by 14 round cast iron columns held in place by diagonal and horizontal wrought iron braces.

Kennard owned the Falkirk ironworks where the iron castings for the Viaduct were produced and shipped to Newport then by canal or railway to Crumlin. Most if not all of the wrought iron was made at the nearby town of Blaenavon by the Blaenavon Iron and Coal Company, which his father R. W. Kennard MP established. Kennard had an assembly plant especially built at Crumlin called the "Crumlin Viaduct Works" where the various sections of ironwork were put together. He also had Crumlin Hall built, where he resided and from where he could view the progress on the Viaduct.

Before work began in the summer of 1853 boreholes were drilled into the valley floor to establish the nature of the sub-terrain. It was found that there was a good foundation consisting of compacted gravel to a depth of some 12 m.(40 ft.). By the following December just seven months later the first pier was raised into place. There was a ceremony to mark this occasion and it is reported that Lady Isabella Fiztmaurice, the wife of the Chairman of the N.A.H. William Fitzmaurice MP, buried an inscribe cup containing coins of the period into the base of the pier, which from then onwards was called the "Isabella" pier.

The viaduct survived in use, virtually unaltered until 1964. In 1929 it was singled as a safety measure for there were reports of its swaying under the force of wind and moving trains, yet it managed to bear the much increased weight of trains and locomotives to the end. Only the abutments remain and those on the east side have a small visitor centre.