When the plan for Scotrail HSTs was first announced, I was delighted, as IMHO these trains, despite their age, provide a far more pleasant travelling experience than any type of DMU, past or present. But clearly the challenges of firstly upgrading them to modern requirements (eg sliding doors) and now maintaining them in service were greatly underestimated, if not unknown. Of course they will have to be replaced, and it is surprising that, getting on for 3 years since Carmont, there appears to be no concrete plan.
But one would have to ask ASLEF, what suddenly makes these trains unsafe to drive from August onwards; if the issue is simply that older trains should not be allowed to operate, what is their policy regarding say Class 318, 37 years old and counting, no replacement plan AFAIK; or 156, 34 years old and counting, also no replacement plan other than the one diagram which will be replaced by an EMU when Barrhead goes live, and a few more when East Kilbride gets its wires; The replacements for which may well be equally ancient EMUs!
Should the HST fleet be 'blacked' from August onwards there is no possible way in which Scotrail could maintain its present timetable with its existing fleet, and severe service cuts would be necessary, perhaps involving withdrawal of service from entire routes such as the Far North and Kyle lines, to allow the Class 158 sets so released to work Glasgow and Edinburgh to Aberdeen and Inverness routes. Hopefully it will not come to that and the existing mitigations to avoid a repeat of Carmont, such as automatic speed restrictions when heavy rain is forecast, will be agreed to be sufficient.