With the collapse of the Penmanshiel Tunnel during March 1979 the East Coast Main Line was subjected to major timetable alterations due to closure between Berwick and Dunbar. At the time of the collapse work was into its seventh week of enlargement of the tunnel by lowering the invert by excavation in a programme then being carried out on this crucial main line for projected new rolling stock.
Services from King's Cross terminated at Berwick, whilst bus shuttles operated between Berwick and Dunbar, and train shuttles then worked the final section between Dunbar and Edinburgh. Only selected daytime trains worked throughout, re-routed from Newcastle via Carlisle, though all overnight services were allowed to traverse the route via Carlisle. Deltics operating the overnight sleepers from King's Cross found themselves marooned in Scotland for the day until a return could be made on the evenings south-bound sleepers. Thus the class were put to work operating the shuttle services between Edinburgh and Dunbar and on the 27th July 1979 55021 (D9021) "ARGYLL & SUTHERLAND HIGHLANDER" restarts prior to departure from Dunbar with the 1F10 14:43 service for Edinburgh. A second member of the class, 55008 (D9008) "THE GREEN HOWARDS", waits behind with what would be the next service for Edinburgh. The section of line via Penmanshiel Tunnel remained closed after the tragic collapse, which killed two workers, and the ECML was re-routed via a cutting around the base of the hill which opened in late August, five months after the disaster.