It is nothing short of preposterous that any company should try to run a 7 day business with staff only contracted to a 6 or even 5 day week.
The majority of organisations (eg the NHS) have rotas and you have to do your turn. My daughter is a "junior doctor" (7 years qualified) and she has to do her nights/weekends/public holiday shifts as part of her contract.
Agree entirely, I feel that enforcing a significant change on existing staff would be unreasonable, and possible contrary to employment law.
I have however previously suggested that all new staff should be required to work weekends, there is nothing unreasonable about this if the requirement is made clear at the recruitment stage.
There would still be a need for some voluntary overtime working by existing staff for some years, but at least it would be a steadily reducing problem rather than ongoing and perhaps worsening.
I would also suggest that if existing employees apply for a new role, that the new role should require weekend working. Anyone who considers this to be unreasonable need not apply.
A more radical proposal might be to allow existing train crew who are about to retire, to continue part time work at weekends only, for say a year.
Todays increasingly busy railway is not the place for very elderly workers, but I also doubt that a driver or train manger suddenly become unfit/unsafe on a certain birthday.