It will depend on how much the railway is integrated, and whether organised into geographical regions or business sectors.
As TOCs get merged, there will be fewer HQ jobs (MD, FD, HR, Legal, Marketing, Customer Services etc).
Front line jobs will probably be less affected unless services are reduced or combined.
Jobs currently with DfT might well be affected, and not everybody will want to move to Derby.
Merging TOC and NR functions is rarely discussed, but might be where some duplication will be removed.