They did stop running on Sundays for a while, as there were enough 555s to run the service. However, due to the reliability issues affecting the 555 fleet, they seem to have been given a stay of execution and have been seen out more often, including on Sundays. Last week, for example, there were 2 Metrocar sets out on Sunday.
There's a chance, but I suspect it's going to be rather unlikely. In addition to the normal Sunday service, there's also Network Rail engineering work between Pelaw and South Hylton (drainage works at Sunderland), which means that fewer trains will be needed on the Green Line (Airport - South Hylton), as they'll be terminating short at Pelaw.
- Ordinary weekday: 12 Green + 16 Yellow (28 trains)
- Ordinary Sunday: 10 Green + 13 Yellow (23)
- This Sunday: 7 Green + 13 Yellow (20)