Well can someone please post a summary in plain English so those following the thread can decide if it's a legitimate dispute or not?
Scotrail had a shortage of traincrew, both drivers and guards, due to factors including under-recruitment as well as outstanding training.
They reached an agreement with the RMT & ASLEF whereby members would be paid 'time and a half' (rather than just 'time') for Rest Day Work, i.e. working on their 'days off'. This was to incentivise people to volunteer, to ensure the service was fully covered.
That agreement stipulated that it would only last so long as there was a shortage of traincrew in the relevant grade.
To a lot of people's surprise, Scotrail recruited and trained enough guards that it was no longer necessary to have lots of RDW to cover the service. As a result, Scotrail have stopped paying guards time and a half for RDW.
However they are still paying drivers time and a half, because there still aren't enough fully trained drivers (as the training takes longer and has been more heavily affected by Covid).
The RMT are undertaking this industrial action, involving the withdrawal of labour on Sundays (which are contractually committed RDW), because they are unhappy that guards are receiving time whilst drivers are receiving time and a half.
Is that a "legitimate dispute"? Well, that's ultimately in the eye of the beholder.