Electrification
The Tyne and Wear Metro uses overhead electrification at 1500 volts DC. This is incompatible with UK mainline electrification, which uses 25,000 volts AC. Additionally the reconstruction of the station to allow the Metro services to call here now means that Mark IV coaches, now cannot call at the station, even if diesel hauled, as they are out-of-gauge.
Read more about this topic: Sunderland Station