Geoff_184 has an excellent sequence of videos that clearly demonstrate the impact of the “missing link” at the new Newmarket station.
As Geoff explains, two western line SA sets, one arriving from Britomart into platform 1, followed by another departing platform 2/3 for Britomart. The latter train had a six minute wait at the station because although it had been ready to depart two or three minutes earlier, it had to wait for the inbound train to clear the single track.
Had a simple crossover link been built from platform 2/3 to the Newmarket Down main, this train could have departed earlier, then changed tracks at the Sarawia Street crossovers, or alternatively it could have been berthed at platform 1, with the inbound berthing at platform 2/3, which would see these two trains avoid each other completely.
Further complications arise at Newmarket when four trains arrive simultaneously at 17:40. A scheduling change is definitely needed.