Said to be the earliest surviving public railway viaduct in the world and certainly the oldest in Scotland, the Laigh Milton Viaduct (once known as Milton Bridge or the West Gatehead Viaduct) crosses the River Irvine, and the border between South Ayrshire and East Ayrshire, a quarter-mile (0.4 km) south southeast of Laigh Milton Mill and a half-mile (1 km) northwest of Gatehead. This A-listed four-span masonry bridge was designed by engineer William Jessop (1745 - 1814) and built 1809-11 by mason John Simpson at a cost of £4000 to carry the Duke of Portland's horse-drawn waggonway which transported coal from his mines west of Kilmarnock to Troon Harbour. An early George Stephenson steam locomotive was trialled on the line in 1816 and, later, the railway also provided a passenger service.
The viaduct is 82.3m (270 feet) in length and by 5.8m (19 feet) wide. Its arches each span 12.2m (40 feet) and the viaduct crosses the river at a height of 7.6m (25 feet). It became redundant in 1846 when the line was upgraded and re-aligned to cross the river on a new wooden viaduct just upstream. In turn this was bypassed by another viaduct, yet further upstream, in 1865, which still carries the railway today.
By 1992, the Laigh Milton Viaduct had become dangerously unstable but its historical value was recognised by the Institution of Civil Engineers. Having been purchased by the Laigh Milton Viaduct Preservation Trust for the nominal cost of £2, it was restored 1995-96 by Barr Ltd. at a cost of £1.1 million. To enable the work to be carried out, the river was temporarily dammed and the viaduct had to be partially dismantled. It was then consolidated, with an internal steel framework installed to support the arches, and rebuilt with defective stone-work replaced, re-pointed in lime mortar and metal railings installed inside the parapet to enable the public to walk across in safety. This work was funded by a variety of public bodies and awarded a conservation award by the Saltire Society for the "skill in restoring a structure on the verge of collapse". Interpretation plaques were erected and a section of replica railway installed, although some of the original stone sleepers can still be found in the river bed beneath the bridge. The viaduct was re-opened on 20th October 1996 jointly by Provost Robert Campbell of South Ayrshire Council and Provost Robert Stirling of East Ayrshire Council. The structure is now owned and maintained by East Ayrshire Council.