As with any such claims, there are inevitably a few asterisks. However, the Laigh Milton Viaduct is undoubtedly a contender for the title. An important part of railway history in Scotland, the impressive 270-foot-long, four-span structure was constructed in 1809 as part of the Duke of Portland's "horse-drawn wagonway."

It was built to carry coal from mines in Kilmarnock for sale to Ireland and beyond through a new port at Troon. The "railway" used horses to pull the wagons and used L-shaped plates to guide the trains. Today you can see a modern reconstruction of the original track on the bridge itself. An early George Stephenson steam locomotive was trialled on the line in 1816 and, later, the railway also provided passenger service.

The viaduct closed in 1846 when the railway was realigned to ease the sharp curves for the new-fangled steam locomotives. The viaduct was restored in the late 1990s and visitors are now able to walk over the structure.