Via Longrider who spent some time in National/Network Rail and has some more depth on the matter, the Scottish railway signallers are striking for 72 hours because they don't like a proposed change to promotions. Let's leave aside the cynicism of choosing 3 days across Christmas as the strike dates, and look at the underlying grievance.
Contrast the Guardian's take on the reason for the strike, which is far from clear:
The Rail, Maritime and Transport (RMT) union claims Network Rail managers have ripped up an arrangement which had been in place for more than 30 years where staff were "slotted" into a post when it became vacant.with Longrider's much clearer explanation, that currently promotion is based on seniority and that Network Rail have the temerity to demand that some measure of competence be substituted for this:
Network Rail said the union wanted a signaller removed from his post before it would negotiate. It said resolving a dispute before the Christmas rush was made "impossible" by the stance of the RMT.
The unions didn’t much like the idea of competence based promotion back in 1994 and it seems they still don’t. It does surprise me that there is still dispute about it getting on for twenty years later, though.
What's really concerning about this is that there is nominally no financial impact on the union membership as a result of this policy change - the same posts exist for the same salaries, and union members occupy them. One union member's loss is another's gain? So why strike? It seems pretty clear that the union is very sensitive about any toe-in-the-door relating to paying or promoting people based on any form of measured performance. Heaven forbid that any unionised workforce should be paid by result.