By the late 1840s, you could already travel from London almost to Berwick by train, and from Edinburgh south to the same town by another line. The Tweed at Berwick was the missing link - the gap that kept the East Coast route from being continuous. The York, Newcastle and Berwick Railway, formed in 1847 through the merger of two companies controlled by the financier and politician George Hudson, was charged with closing it. Hudson was a force of his era, the so-called Railway King, and within a few years his empire would collapse in scandal. But by then his trains were already crossing the Tweed.

Robert Stephenson, son of the pioneer George Stephenson, designed the viaduct. His brief was demanding: cross 659 metres of valley, carry a heavy main-line railway 37 metres above the river, and do it in stone befitting Berwick's medieval walls. He gave them 28 semicircular arches, each spanning 60 feet, the stonework dressed and the soffits lined in brick. From a distance it looks Roman - a long pale arcade striding above the dark water of the Tweed, with the red-roofed huddle of Tweedmouth on the south bank and Berwick's ancient walls on the north. Construction began in 1847. Three years later, on 29 August 1850, Queen Victoria's train crossed the new structure, and she stopped at Berwick to perform the opening.

The Royal Border Bridge is Grade I listed, meaning it sits in the top tier of British heritage structures - the same category as Westminster Abbey and Stonehenge. But it is not a monument. It is a working piece of national infrastructure. In 1989 it was electrified as part of the wider East Coast Main Line upgrade; between 1993 and 1996 it underwent its first major repair work in over a century, a Railtrack-led project part-funded by English Heritage. In 2016, on the bridge's 160th anniversary, Northumberland County Council finished installing colour-changing LED lights along the arches, turning the viaduct into a slow nocturnal lantern that switches palette across the seasons. From the right vantage point in the town, the bridge changes colour while the trains rumble overhead.

There is a path along the river under the arches, used by walkers and cyclists on the Tweed bridges route. From down there, the structure looms in a way photographs rarely capture - 37 metres of clean stone above your head, the underside of each arch a curved brick vault, the whole thing humming faintly when a train passes. The Tweed itself is wide and slow at this point, fed by the Whiteadder a few miles upstream and emptying into the North Sea just below. Salmon still run the river. Cormorants nest on the bridge piers. And the trains, several an hour, still make the run between two capital cities, crossing a bridge named for a border that lies, as it always has, just a little further north.