Station staff can't be forced to work Sundays or move around different stations, if this wasn't in the existing contract they signed; that's why there are GPR positions, to cover multiple stations. What if they don't own a car and can't reach the station they're asked to cover in time, or can't get home that evening, by using public transport?
There are employment rights which prevent staff from being forced to work Sundays if it was previously optional overtime. Obviously opting out of working Sundays would be reflected in a reduced pay offer for the individual, or no Sunday pay at all if they are 'committed Sundays'.