Following the opening of Sunderland Central station, NER and LS&SR services were diverted away from the earlier termini and focussed in one location for the first time. Following the completion of the improvements of the 1870s, the Durham Coast Line could be operated as a through route but, having not been constructed for such a purpose, doing so it remained challenging. One major issue was that any services running between Hartlepool and Sunderland still had to ascend or descend both the 1:44 incline at Seaton Bank, and the aforementioned incline at Hesleden Bank. To alleviate this, the NER developed plans to construct a new, more direct, line along the coast. Construction of this new line between Seaham Colliery on the LS&SR, and Hart Junction on the ex-HD&R route was sanctioned in 1894–95. However its construction was contingent on the NER purchasing the Londonderry Railway's Seaham to Sunderland route and this was not agreed until 1900 when the NER agreed to pay £387,000 for it. This new coastal line had to cross the denes at Hawthorn, Castle Eden and Crimdon, each requiring a substantial viaduct; one of the most imposing of these, Denemouth Viaduct (spanning Castle Eden Dene), is from ground to rail level, and consists of 10 arches, each with a span of . The construction of these viaducts required the opening of a special brickfield and, in the case of Denemouth Viaduct, the creation of a temporary cableway spanning valley. The line opened on 1 April 1905, with new stations constructed at Blackhall Rocks, and , to serve the new villages that had been created to house workers from the new coastal collieries which came into existence thanks to the provision of the railway. The NER became part of the London & North Eastern Railway, as part of the 1923 grouping. Despite them having already begun to experience a decline in traffic due to competition from road transport, the LNER did provide some improvements to the line. One notable improvement implemented by the LNER was the electrification of today's DCL between Newcastle and Pelaw Junction, using the 600 V DC third rail system, as part of an extension of the Tyneside Electrics system to , inaugurated on 14 March 1938. The other notable improvement during this period was the opening of additional stations at on the NER-built Seaham to Hart line in 1936, and at Seaburn on the former