Signalling is always the last discipline to complete and hand over, simply because it's not possible to finally connect up all the external track circuit bonds to the rails and the cables to point machines until the civil and permanent way work is complete. Interlocking and panel/vdu etc is all pretested, either well in advance in the office for computer based systems or earlier during the work while the heavy trackwork is being done, and using test switch panels to simulate the exterior equipment if a relay-based system. Teething problems with some exterior equipment may not become apparent until that final changeover, and it's possible there can be other underlying causes, say a poorly-aligned new switch preventing a point machine making detection, or a mistake in traction bonding shorting out a track circuit. No excuse clearly but illustrative of the tension in that final stage of the work.