Is this the derailment which happens at the very end of the scenario, when heading back to the Edge Hill Shed? If so, there's a work around. After you start reversing from Rathbone Road 02, keep reversing until a red signal stops your path. You can then set the manual points so that you can drive through the shed up to the Edge Hill Shed Headshunt, from where you can reverse normally to the spot the scenario wants you to go to.
