Think of working Sundays as overtime in the same way that office workers might be asked to work midnight to 6am. Sure, the money might be good, but would you do it on a regular basis if you either didn’t want to, or didn’t have to?
The industry is, broadly, unionised. The RMT is the trackside, conductors and office staff etc, whilst the drivers are ASLEF. Management are TSSA (who,indeed). Those unions make the trains safer, get involved on efficiency improvements (it would still be steam trains running if they didn’t) but they won’t be pushed around. The rest of the U.K. workforce has been systematically beaten down for decades. Not the rail unions who, generally, get cost of living pay increases and have had their hours per week negotiated over the years down to around 36 hours per week. Above that it is at premium rates, including non compulsory Sundays.
Throw in failed privatisation, years of infrastructure issues, changes of government priorities (HS2 etc) and we are where we are.It will improve but not overnight.
Great British Railways is the new idea. So too is integrating bus and tram network timetables with railway timetables. Look at what goes on abroad on transport networks and there is a lot we could learn.