I’ve known about the Causey Arch for as long as I’ve known anything about railway history.

It always appeared in the textbooks, often in unlikely-looking engravings, but was not much visited because until the 1980s it was neglected and not very accessible.

It’s an outstanding piece of industrial archaeology because it was, at the time it was built, 1725-26, easily the longest single-span bridge in Britain, 102 feet between the abutments and eighty feet above the Causey Burn.

It can also claim to be the world’s first railway bridge, carrying a wooden tramway conveying coal from Tanfield Colliery to the River Tyne 5½ miles away.

An earlier bridge had collapsed as soon as it was completed.  Indeed the stonemason of the existing arch, Ralph Wood, was so nervous about the strength of its replacement that he killed himself before it was completed.

A reproduction of one of the wagons is displayed at the site:  these wooden wagons, with wooden wheels running on wooden rails, were controlled by a disconcertingly basic wooden brake.

Friction sometimes caused the wheels and brakes to catch fire.

Wagons loaded with 2½ tons of coal rolled down the “main-way” grade by gravity, retarded by horses, which hauled them back to the pit empty up the opposite track, the “bye-way”.

Nine hundred wagons a day traversed the line – one every twenty seconds crossing this great masonry arch which seems to have had no parapet.

By the magnitude of the arch and the volume of its initial traffic we can judge how much money was to be made from Durham coal in the eighteenth century.

Its heyday was shortlived.  It declined after Tanfield Colliery caught fire in 1739.

Though it was listed grade I as early as 1950, it was neglected until the local council restored it and developed its surroundings in 1980.

Since the Tanfield Railway began regular services to the station beside the Arch in 1991, it has become an easy and popular focus for walks in the area.