The link to Mr Bray's letter in The Courier is at:
http://www.thecourier.co.uk/opinion/readers-letters/st-andrews-rail-line-would-never-be-viable-1.912987
First of all, the aim of the campaign is not to 'reinstate the rail line between Leuchars Junction and St Andrews.' A new indicative route, a link to which is available on this site, has been identified, of a branch line joining the main line some way south of Leuchars, allowing direct services both to Edinburgh and Dundee. There would therefore be no need to change at Leuchars, thus saving on average 10 minutes per journey.
Mr Bray is correct, though, when he says that 'the service would have to be fast and very frequent'. As the study makes clear, the new alignment enables speeds twice as fast as the Victorian branch line, reaching the main line in 5 minutes rather than 10. Hourly services both to Edinburgh and to Dundee are proposed, though there is actually capacity for 2 trains per hour north.

Finally, Mr Bray is concerned about viability. The Tata report says that, unlike most services, a St Andrews service would operate at a profit.