In 1844, after the West Durham Railway extended from a junction with the Clarence Railway at to Crook, the S&DR extended the BA&WR from Bishop Auckland along the river valley to Witton-le-Wear, and then into Crook. In 1845, the S&DR came to an agreement with the Derwent Iron Company to sub-lease the southern section of the former Stanhope and Tyne Railway. It extended the line from Crook to and then to Blackhill. That line was opened as the Weardale Extension Railway (WXR).