To be fair to @MilesBriggsMSP there may be quite a few MSPs, from various parties, making populist noises as elections approach. Growth of Edinburgh, need to encourage sustainable travel means case for using Southern Sub for passengers is stronger, but it will require imagination https://twitter.com/nigelbagshaw/status/1327304262603444224

Certainly seems to me idea of just running passenger services round the line to Waverley/Haymarket will never be viable option, for a number of reasons:

(1) South Sub used to be part of network of local rail routes serving many places within Edinburgh, but now it is only likely to be more convenient than bus/tram/bike for ppl living near the stations, travelling to places near Haymarket/Waverley, partic as frequency will be less

(2) In partic, South Sub unlikely to be best way for most of going by public transport to new ERI, or to Edin Uni buildings at Little France or Kings Buildings

(3) also, there will be a capacity problem of running more trains through Haymarket - Waverley, partic east of latter because of removal of 2 out of 4 tracks through tunnels, after electrification of East Coast Main Line

So there will need to be more imaginative made of South Sub. One would certainly be tram-trains as suggested by @nigelbagshaw but also possibility of using route for 'crossrail' type of services, not using Haymarket-Waverley section @_MMaritima

That could provide direct rail links to Edinburgh Park (big traffic problems there), Heriot-Watt from South & East Edinburgh, East Lothian, Midlothian/Borders. Certainly idea ought to be taken further. And much better use of cash than £160 million on new Sheriffhall roundabout