The Welwyn Viaduct (or Digswell Viaduct) carries the East Coast Main Line over the River Mimram and is a locally important landmark. It is located between Welwyn Garden City and Digswell. It is just to the south of Welwyn North railway station.

The viaduct is around 475 m (1,560ft) long and comprises forty arches of 9.1 m (30ft) span, and is 30 m (100 ft) high from ground level to trackbed. It is built of brick fired from brick clay quarried on site during construction, and took two years to build, including the construction of embankments at both ends which required the movement of around one million tons of earth by human and horse power. It was designed by William Cubitt and styled after a Roman aqueduct.

It was originally opened by Queen Victoria on 6 August 1850, but she was so frightened by its great height that she refused to travel across it. The train carrying her had to stop, whereupon she left it and entered a horse drawn carriage to travel the length of the bridge on the ground. She then re-entered the train at the other end of the viaduct and continued her journey. It carries the East Coast Main Line, which has to narrow from four tracks to two to cross the viaduct, making it a bottleneck restraining capacity over this strategic transport route. Several ideas to overcome this limitation without damaging its essential historic character are  discussed from time to time.