It would be interesting to see what extra services above what was run Boxing Day 2019 they attempt to operate on Boxing Day 2021 as the majority of the major stations are either operated by other TOC's or NR.Transport for the North signed off the decision to not run anything Boxing Day this year, with the Covid effect driving the decision. There will be services again next year, and more than was attempted in 2019.
Aside from engineering work, from memory this was the reason precluding Northern from running into the likes Manchester, Preston, Leeds etc.