Altnabreac will now come in with his golden rules of reopenings and point out that there's never going to be a business case for reopning Dumfries to Stranraer. How much would it cost, and how many people would use it? The Borders route reopened because it would allow Midlothian and the Borders access to employment in Edinburgh. Access to employment in Glasgow and Edinburgh is the justification for all the reopenings in Scotland, something that Dumfries to Stranraer wouldn't do at all.
The way that Transport Scotland plans capital projects is that they focus on the outcome they want, not just a technical detail. For instance, if they looked at south west Scotland they would identify increased access to employment in Glasgow as being one of the key goals. Then they would look to see what projects would deliver that aim, including both rail and road projects. In this case, the idea most likely to go ahead would be to electrify from Ayr to Girvan and extend one of the Ayr express services every hour. Electrification and linespeed improvements (where possible) on the GSW line would be another likely outcome, so that a Dumfries-Glasgow commute would be bearable. Since the planning process doesn't set out technical solutions first, such as re-opening a certain railway line, it leaves them open to new ideas. For instance, it might be found that the best way to speed up Dumfries-Glasgow journeys would be to build a chord between Lockerbie and Dumfries and run a WCML service. Another option could be to improve the connecting service to Carlisle and then have people change there onto existing and future express services up to Glasgow and Edinburgh.