Edit: As a side note, has anyone else noticed that particularly on an evening, Washington and Chester specifically are running a skeleton staff? Saltmeadows and Arriva run everything local in Washington and (I may have slipped up) only the X1 runs into the night from Washington.
Chester is in a similar position, with only a couple of routes working late.
Stanley, Deptford and Sunderland Road run a few more routes. Not sure about Winlaton, but guessing there are a few services out of there late on.
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Yes I've noticed.

Arriva operate the M2/M3 during the Evenings Mon-Sun between Concord and Birtley, whereas GNE operate between Concord and Heworth - Arriva/GNE Interwork the Service on a night and Saltmeadows operate the W5/W6 using Nexus Branded Solo SR's, Basically it's down to running costs and the Fact Nexus own the Contracts for these services on a Night etc.

As for the X1 that runs late on as some people may work until late, and also you have the clubbers aswell. I've never understood why there was never an NX1, Although the N56 goes through Washington Galleries on a Night, it's the same with the M1 from Heworth/Houghton, it would save passengers £15-£20 on Taxis Home