The Causey Arch is a bridge near Stanley in County Durham, northern England. It is the oldest surviving single-arch railway bridge in the world, and a key element of the industrial heritage of England.
It was built in 1725-26 by stonemason Ralph Wood, funded by a conglomeration of coal-owners known as the "'Grand Allies'" (founded by Colonel Liddell and the Hon. Charles Montague) at a cost of £12,000. Two tracks crossed the Arch: one (the main way) to take coal to the River Tyne, and the other (the bye way) for the returning the empty wagons. Over nine hundred horse-drawn wagons crossed the arch each day using the Tanfield Railway.
At the time the bridge was completed in 1726, it was the longest single span bridge in the country with an arch span of 31 m, a record it held for thirty years until 1756 when a bridge was built in Pontypridd, Wales.