In itself, a reopening to Grangenouth could allow extention of Glasgow-Falkirk Grahamston services, as suggested. But this may not prove popular as it is a much longer alternative than the direct Falkirk High Train (Edinburgh-Glasgow).
Edinburgh-Grangemouth would undoubtedly be a more popular and sensible option for the town. A direct line would serve Polmont and Linlithgow, which could allow more fast Edinburgh-Glasgow services. However, I'm unsure if Linlithgow and Polmont would want to sacrifice any direct trains to Glasgow.
I think this could prove tricky. The business case may be there for reopening, but the actual route is going to cause problems.
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But there are other Scottish improvements I'm impatient for, and they needn't all be costly! They are: 1. greatly reduced journey times between Stirling - Perth and both Edin & Glas, and, 2. stopping services that connect the Scottish and English towns on the ECML between Edin & Newcastle,
Stirling to Perth could (and IMO should) be improved by electrifying the line up to Perth when they're doing Stirling electrification (2015ish?). This would in itself allow faster journey times. While they're at it, they could improve the line as well at minimal extra cost and disruption.
Edinburgh-Newcastle stopper is a good idea; I've always thought it would be feasible. But politically, it would be a nightmare given devolution (let's just say I don't expect Alex Salmond to happily pay for any of this). The other problem is passing loops, are lack of them, for the faster services.
Borders will actually be a good rail link when it opens; it is serving pretty large conurbations (some of the largest in Scotland without rail stations). But I do worry that it's being done on the cheap. They should have learned their lesson from Alloa; rail is gaining popularity and you should allow for a lot of growth. I strongly believe it would also be a good opportunity to electrify the line and provide a second wired route from Edinburgh-Carlisle, potentially helping to clear the WCML.
As long as they can upgrade to double track as and when necessary, I think they may get away with it.