Demolitions might be necessary to achieve doubling. Obviously whoever spec'd the line at the time must have left. That said perhaps passive provision for double track would have been better at the design stage..
The land along the whole line was bought up, giving a footprint that allowed for double track along its length.
How's wholesale rebuilding of certain sections, most notably the viaducts over the River Esk and at Hargreen, as well as Newtongrange station, however it is possible inside the land footprint owned by NR.
However, that doesn't really answer the question of "is it work rebuilding the whole thing?"
I agree wholeheartedly that if it were open today, we wouldn't be thinking of closing it, however the timetable runs well enough with 2tph at present, and wiring will speed those journeys up and make timekeeping easier, that redoubling seems a bit redundant.
Extending to Hawick seems a reasonable suggestion, and we'll probably see it done in my lifetime, however beyond that is a basket case, as this report basically proves.
If the report has to justify rebuilding as an environmental benefit to the farmers in the Borders by reducing air pollution, or that providing a 2tph service to Carlisle will somehow kickstart an industrial powerhouse in the Langholm valley, then personally I remain unconvinced.
(With HGV truckstop/ Irish Sea border control of course- a true Borders Project)