Great Western Coffee Shop

Would they like some 165/6s ?   Five abreast seating to snuggle up and keep warm.......................

https://www.bbc.co.uk/news/uk-scotland-glasgow-west-68241768

More seriously, it cant be that hard to fit some mains powered electric heaters, to preheat the train prior to departure. Either standard convector heaters, or immersion heaters in the existing engine cooling system. This later option would preheat the engines and assist reliable starting.
Starting with a warmed train, then the engine derived heat should suffice, especialy as the engines would be working and not simply idleing.
I think that they have electricity in much of Scotland now.

Not sure that would help much.  Train interiors get cold (and warm) very quickly in the sort of temperatures described.  A toasty train before departure would soon become very cold again.

I remember Turbos taking many hours in service with engines running before the heating started to work.

Better HVAC systems are the answer, as have now been fitted to the Turbos, though they are likely to take more power and therefore have an effect on acceleration.

I think that starting with warmed train, or better a hot engine coolant system would help considerably. When the engine is working hard almost unlimited heat should be available from the coolant. It is idling prior to departure that produces little heat.
Any electric cooling fan should be automatically disabled until the coolant is hot.

Yes, I don’t doubt the science behind that, but just the reality - which is that a lot of units built around that time had a poor heating systems installed and poor insulation.  It matters not how hard an engine is working if the system of pipes, radiators, thermostats etc that feeds into the saloons  isn’t very good.

As the article says, these trains didn’t heat up the whole day, when the engines would have been powering for a considerable percentage, and that tallies with my experience on the system originally fitted to the Turbos, in less extreme temperatures.

Contrast that with a modern unit like the IET which I’ve watched heat from 5 degrees to 18+ degrees in less than half an hour of idling from being left switched off overnight.

Then of course if you don’t leave it idling at the terminus…. ;D