If I recall correctly, the Benton North West Curve on Tyneside almost fits this catagory. It was built during WW2 as a diversionary route so that trains could reach the main line north from Newcastle if the normal line through Heaton was interrupted.
I really remember it from the early 1960s, it was always rusty and Benton North box, where it joined the main line, about ½ mile from Forest Hall, was alwys switched out. It was occasionally used for engineering diversions and I do have a memory from early childhood of seeing a double headed tank engine + what was probably an A3 running through South Gosforth on a rake of blood and custard coaches.
When the Tyneside area was resignalled, around 1962, it all came under the control of Benton power box. From then on, a late evening ECS from Morpeth or Alnmouth to South Gosforth sheds was routed that way to keep track circuits operational and, later, an odd Alnmouth stopping DMU was routed over it, calling at Benton en route.
The line closed as a through route when the North Tyneside lines were converted for Metro operation, but was used for a few years as a stabling and turn-back facility.
At the time, I remember hearing that the North to East curve at Morpeth was funded as part of the Metro scheme as compensation for loss of the diversionary route through South Gosforth.
