That area of Smallcleugh over recent years, seems to be on the move, so it is not really a surprise that the side of the shaft decided to join in!
Nick, I think you might be able to bolt a couple of rails together, to give you the extra length you are going to need!
I think you would have to take rails out of the main horse level.
Probably best to cut off the nuts on the rails, using a battery angle grinder.
You might also be able to brake the nuts, off by bashing them with a really hard cold chisel. Good luck!