If there were any justice in the world, Nemmie would be immortal and it is amazing that so many of us think the first and oldest SW would be the last to leave, but the realist in my says this probably won't be the case.
Totally agree with Benjsh - when the time comes, just retracing it with the same layout.
Hopefully with the pit dug and the plans still on the B and M's database, it would cost a fraction of an original design as it is the only way that Merlin would do it.
That said, would an identical track replacement of Nemmie be Nemmie?