I too do hope that the line does get extended from Tweedback to Carlisle at some stage in the future. However I can see lots of reasons for this not happening - the biggest one is the cost of building it in the first place coupled with the lack of revenue expected due to scarcity of population en route.
Now if the line was in existence, however, then there would be additional benefits to the network as a whole re as a diversionary route in times of blockage elsewhere. The line would also act as a feeder route into both Edinburgh & Carlisle thereby creating new journey opportunities, and would thus also help social inclusion in the South Borders/North Cumbria area. This line would also bring inward investment to the area. (This is similar to the function played by the GSW line between Carlisle & Glasgow).
Now if this railway was to be built "on the cheap" but obviously to the high standards expected of a main line route - then this could be a winner?
My solution is this, and remember that when the line was originally built in the 1860's that in the main it was built by raw muscle power (horse & man) alone. The Victorians built many a great structure with little more than a few calculations, a pick, & a shovel!!
So, firstly, we have to organise large groups of labour on the cheap. There are various sources for this but I suggest using category C prisoners and the able bodied unemployed (known as the idle poor in Victorian Britain). Judges should be given the power to sentence convicts to "hard labour on the railways", and legislation should be passed to make it a condition that the idle unemployed should have to work for their money. (NB - I do not want to be seen to persecute hard working people who have worked previously but who find themselves unemployed through no fault of their own), this work would also be available to volunteers (who would be paid however) seeking new skills in civil engineering and a challenge. Heritage railway operators could also get in on the act by volunteering their time & expertise for future running rights on the reopenned route.
Secondly, we need to find people with the skills for the major civil engineering projects for a scheme of this size. I suggest using the armed forces engineering regiments as well as the engineering departments of the various colleges and universities that we have in this country - this work will provide sound practical experience for these soldiers/students.
Thirdly, we require qualified engineers to oversee the whole operation. Recently retired ones that are willing to give up some free time to this worthwhile enterprise should be sought. The line would not be handed over to the operating department until such times as stringent safety checks are carried out
I think this plan, which I worked out down the pub last night, could be a goer if the Government has the balls to change the legislation required. I feel this type of project could help install a sense of pride back into certain sectors of our society. The local communities would ultimately gain by being reconnected to the rail network with all the benefits mentioned above. The workers would benefit by receiving new skills & opportunities, and also some pride in giving something back to the community. Society in general would also benefit - benefits all round.
I would suggest that this and similar schemes are only suitable where there is no commercial reason for reopening a rail route (we would not want to use "free labour" on projects that would take jobs away from other workers). There should however be a reason for running services on the reopened line be they social, commercial or indeed both.