Joseph Mitchell was the engineer in charge of the Highland Railway's northward expansion. Four years earlier, he had built the Dalguise Viaduct over the Tay with a 210-foot span, then the longest of its kind on the network. For the Kyle of Sutherland he needed to do better. The site was awkward - a narrow estuary with steep banks, no easy place to drop a midstream pier without obstructing the tidal channel. He answered with a single wrought-iron truss spanning 230 feet, supported on stone piers at either bank. To approach the truss, masonry arches of semicircular form were built up to the level of the deck - two arches on the south, three on the north.

What makes the Shin Viaduct distinctive among its contemporaries is a small but important choice. The deck that carries the railway track sits on top of the truss girders rather than between them. This 'deck-truss' arrangement keeps the rails high above the river and offers passengers an unobstructed view of the Kyle as the train rolls across. Most British truss bridges of the same era buried the train inside the steelwork. From the riverbank, looking up, you see the underside of the trusses with the trains running above them like something on a tabletop, the only sign of activity the rumble and the brief shadow.

The Far North Line is one of the most scenic railway routes in Britain, running 161 miles from Inverness to Wick and Thurso. The Shin Viaduct is a small but indispensable link in that journey. Without it, trains would either have to detour many miles inland up the Kyle to find a crossing or descend to a low-level bridge that would be difficult to maintain. Instead, twenty-first century Class 158 diesel multiple-units cross the same Victorian ironwork that carried Highland Railway compartment coaches in 1868. Maintenance and inspection keep the viaduct safe; its Category A listed status protects it from alteration. From a passing train, the crossing takes only a few seconds. Look up the Kyle and you may see Carbisdale Castle on the hill above Culrain, a strange pairing of railway pragmatism and aristocratic theatre.

In 2000 a footbridge was added to the northern side of the viaduct, allowing walkers and cyclists to cross the Kyle of Sutherland alongside the railway. National Cycle Network Route 1 - the long route from Dover up to John o' Groats - now runs over the Shin Viaduct, sharing the structure with passing trains. The footpath is narrow, the wind off the water can be sharp, and the noise of a train passing inches away is startling the first time. It is a strange and excellent moment in the route, a Victorian piece of engineering carrying both ScotRail's diesel units and a cyclist on her way to the top of Scotland.