What's the proposal - in practical terms?

TTfE wants the City to run tram-trains around the 'South Sub' on the existing railway, which runs from Murrayfield and Haymarket via Gorgie and Morningside to Niddrie and Brunstane. The railway is relatively lightly used, but double track, so running 4 or 6 trams an hour looks feasible. For the existing rail route, there would have to be minor upgrades, plus building of the intermediate tram stops, in some cases with access from roads well above or below the line - but nothing complicated in terms of new infrastructure.

At the western end a detailed feasibility study would need to determine the route, as the trams would have to cross under or over the existing Edinburgh-Glasgow main line, to reach the airport tram route on the north side. This is likely to be the most expensive capital works for the scheme, possibly with a flyover like the existing flyover at Saughton. An easterly route leading from the South Sub up to Haymarket is likely to need some engineering but looks possible largely on unused land. 

At the eastern end, more limited infrastructure would be required for the tram route to cross the East Coast railway by rising up onto Sir Harry Lauder Road, before descending to a new tram stop above Brighton Place - currently this land is largely unused and installing track would be simple. 

In effect building the scheme is about building connections at the eastern and western ends - 90% of the route is already railway track.