They used to run a lot of freight on the Waverley line.
This is true, but the line closed in the sixties. Sixty years ago, most stations still had a goods yard, and freight brought in goods and parcels to stations across the country. The original line was built to main line specification, with double tracks and the capacity to fit freight in between passenger services, and most importantly, far more freight moved by rail; road transport just didn't exist to the same extent.
There are very solid reasons why freight isn't the solution to a business case.
Theoretically, you could create demand for timber traffic. Most likely, this would be starting in the middle and heading south, to Carlisle. But if that's the case, then why hasn't a stub from Longtown to Newcastleton or Langholm already been built to serve this? Timber is an incredibly low value commodity, and yes, if you built the line, I'm sure they'd like to use it, however you're not going to create a business case from hypothetical timber traffic.
If we pick a random day, there are three freights scheduled past Auchengray, going from Carlisle to Edinburgh, or vice versa.
There is a daily each way departmental Carlisle NY - Millerhill. This theoretically could, however there's no link from the Borders line into Millerhill at the north end, so you'd need to run round somewhere, probably at Niddrie. Alternatively you could reverse it up the hill for about a mile, but I'm pretty sure the signalling rules wouldn't like that very much.
There is the weekly Torness - Sellafield, which again theoretically could, however again there's no direct link, so again you'd need to run round or reverse somewhere, probably at Portobello, on the Leith Docks branch. This one is easier, as you could run them top and tail.
There is the Oxwellmains - Carlisle cement working, which again suffers with needing to run round. This time there's no loco at either end, so a run round is necessary. Running around at either Niddrie or Leith has its issues, the former you're blocking the platform at Newcraighall and I'm not even sure that the signalling is set up to do that, the latter you're crossing the ECML twice at Portobello junction, which would be a big performance hit.
Next, it is reasonable to expect all freight would be worked by class 66 (cement/departmental), 70 (cement) or 68/88 (flasks). All class 66 hauled workings are subject to severe speed restrictions across a number of weak underbridges, down to 20mph. Currently, although it would be possible to change, again subject to severe speed restrictions, classes 68, 70 or 88 are not permitted over the line.
If you try running a freight in the middle of the day, there's not really the space in the timetable to fit them in. Having to crawl down the single line sections, and the very very simple signalling setup, which basically amounts to single aspect signalling with repeaters, doesn't allow you to run more than the timetable already in place without signalling improvements.
The weak underbridges and the bare minimum signalling are poor in retrospect, because they limit the potential for freight without infrastructure interventions. However, when we say that the only way we got the line through was by fudging the numbers, this is what we were doing. We specified the bare minimum infrastructure for a 2tph service, because completely eliminating any future freight use from the plans was the only way we could ever have justified building the line.
Sure, thats a pretty ****ty situation to be in ten years down the line if a freight company did want to use it, but if we'd needed to upgrade the bridges and signalling to fit more than 2tph of multiple units, the line wouldn't be there for a freight company to use in the first place.
There is a lot of love on here for the prospect of extending it seems. Good lord people on rail forums are cynical, if you applied some of the logic shown on this thread nothing new would ever happen anywhere
There is a lot of love, and there is a lot of cynicism. A lack of business case here doesn't mean that nothing will ever happen. It's important to say that there is a business case in places where there is, and acknowledge a lack of business case in plans like this, so that lines like this don't become a pet project, and fail to ever wash their own face.