Complex sentence: They were not delivered until three months after the line opened, up to that time Festiniog locomotives worked the line, and did so occasionally thereafter if traffic was heavy or the F&BR locomotives were being repaired.
Split into simpler sentences: They were not delivered until three months after the line opened. Up to then Festiniog locomotives worked the line, and did so occasionally thereafter if traffic was heavy or the F&BR locomotives were being repaired.