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Fred Mackintosh argues the large number of trains already using the tracks between Waverley and Haymarket mean it is not realistic to restore the “South Sub” as a circular commuter route linking places like Morningside and Newington to the city centre.
But he says an upgraded South Sub line could become a congestion-busting alternative route for people from southern Edinburgh, Midlothian and East Lothian needing to get to Glasgow.
The Liberal Democrat manifesto for the Holyrood elections includes calls to reopen railway lines closed by the Beeching cuts in the 1960s, with regular reports to parliament on progress.
Mr Mackintosh, who is standing in Edinburgh Southern and on the Lothian list, said: “After we put the wheels in motion for the reopening of the Borders railway, which has been a real success, the Scottish Liberal Democrats believe it has to be full steam ahead on making our rail network the best it possibly can be.
"By carrying out a review of which lines would be viable and then getting them up and running we can deliver a real benefit to communities and the environment. We need to clean up our air and drive down Scotland's stubbornly high transport emissions.”
The South Sub, linking a ring of stations like Abbeyhill, Portobello, Newington, Morningside and Gorgie to Haymarket and Waverley, was opened in 1884 but passenger services ended in 1962 and the line is now only used for freight and diversions.
Mr Mackintosh said as a city councillor between 1999 and 2007 he had been involved in previous discussions about reopening the South Sub and one of the stumbling blocks had always been the lack of track capacity between Waverley and Haymarket. “This means there will never again be a circular heavy rail service linking Morningside to Waverley like there was in the 1950s.”
And he said suggestions of running trams on the South Sub instead ran into the problem that they would have to cross the main Edinburgh-Glasgow rail line, which would be prohibitively expensive.
But he said Network Rail currently had a project which was looking at increasing capacity on the South Sub and also electrifying the line, which he argued opened up the possibility of it becoming a strategic passenger route offering better connections from south eastern Edinburgh, Midlothian and East Lothian to the west of Edinburgh and the central belt.
He said: “Imagine a fast, direct service running from the Borders Railway stopping at Eskbank, Shawfair, Newcraighall, Kinnaird Park, Craigmillar, Newington, Morningside and then running down the Shotts line to Glasgow Central. This would be a real alternative to the City by-pass and trains could whisk travellers direct from Eskbank to Glasgow in a little over 90 minutes and Morningside to Glasgow Central in a little more than an hour, all without travelling into Haymarket or Waverley for the train to Queen Street.”