I think the main issue is Hardengreen Viaduct which is new built and definitely single track. I will be going to Tweedbank later today though so will have a look for any other problems.
Newbattle viaduct is also single track. I was at a crag talk where the NR speaker said that to allow double track given current H&s rules would be prohibitive. If it had been in continuous use as a railway it wouldn't have been an issue.