I can't remember if there were any solid numbers posted the last time this came up. How much of a demand would there be for services on this line if it was upgraded to passenger standards? How much would it likely cost to do so?
There is a notable demand for transport for Glasgow <> Fife, a corridor that is largely dominated by cars - yet even still leads itself to Stagecoach providing a 2 coaches per hour/162 seats per hour service along that corridor via Cumbernauld, Larbert and Kincardine, which at peak times can run full.
Before 2020, there was a peak time run serving the corridor, albeit via the coast: 1N80 0712 SX Kirkcaldy - Glasgow QS and 1N79 1733 SX Glasgow QS - Markinch. Whilst this has now about to become rematerialised in a somewhat bastardised form as a return from Glasgow QS to Dundee via Dunfermline, both run in the evening too late for the main commuter traffic regardless (albeit retaining the same headcodes).
Having a battery-electric service (as the acceleration savings are similar) from Glasgow to Leven running roughly hourly, if permissible, as an extension of the present Alloa service would most likely create more modal shift from car than some in this thread seem to realise.
In theory, could the south curve be built to bypass Charlestown Jn and then one of (if not both) of the two north facing bays at Stirling be resignalled for passenger use such that a peak hour SX run could be used for Stirling - Edinburgh via Kincardine, to give those places a single extra service, or is the Forth bridge already too crowded/other financial reason.