I think a Lockerbie service would be a very popular and viable service. A limited stopper from Lockerbie calling at Carstairs and Motherwell would definitly take the ease off both TPE and existing Scotrail services throughout the route. The diagrams for Lockerbie and Carstairs are really infrequent that it surprises me, given the rising patronage at these stations, that there's no clear indication from Scotrail to provide a service.
Is there the capacity at Central to run it? (Obviously it would require new stock, and that would depend on the BCR as to how likely that is to happen!)