Getting to grips with proper scenario editing (as opposed to hack tests for specific functionality) has brought home to me how important the track direction setting is to the correct working of the AI - and how easy it is to inadvertently get it wrong!
I puzzled for some hours over why the 'supporting cast' of my timetabled scenario between Cambridge and St Ives were not playing ball - before finding a piece of track downline from the AI start point that was set the wrong way. Obviously the scheduler saw it and couldn't resolve and so just sulked ! The clue to why this piece of down line was set to 'up' lay in the proximity to a trailing crossover where I had recently set direction (correctly I might add) for the up line - I suspect that I had accidentally 'dragged' the track selector across the crossover and on the down line without knowing and then set the entire selection to 'up'.
Easily sorted once I found out - and the AI now progress beautifully as my stopper makes its stately journey through the Cambridgehsire countryside !