Signals & Track Problem Fixed ,track Sections Fill Required Whenever Editor Is Restarted

I have a very simple timetable with a single service, used for making runs around my level to check for any problems.
For some time I have been unable to compile this timetable after an editor restart. An error claiming a path cannot be found through to the end point of the service always results until I go into Network Edit/Manage Track Sections mode and press fill again. This happens despite the fact that the track layout has not been changed at all and the previous session was definitely saved.
I wonder if anyone else has had this problem and can maybe suggest a way of making the track sections paths stick the way they should.


Edit: Just discovered a signal where the list of routes was greyed out so deleted it, reinstated it and now all is working again.