Passengers experienced up to around three hours of delays after a compressor failure meant the train's doors could not open.
Northern and Metro services along the line were affected in both directions after the service limped to a halt between Fellgate and Brockley Whins stations at 12.23pm.
A replacement taxi service operated between Heworth and Brockley Whins stations while trains continued to operate between Brockley Whins and Sunderland's South Hylton station.
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Metro operator Nexus tweeted just after 3.30pm: "Service has now resumed between Pelaw and Brockley Whins in both directions. We are operating in this area subject to delays."