As one of the authors of the book, I apologise for not including the information about this. We have a lot of information on private siding agreements which wasn't included, partly due to space constraints and partly because our market research told us that people prefer images to text. That said, it is not clear when the line to GCG closed. The private siding agreement was "deleted" in September 1964, the signal box was closed on 27/9/64. John Smith, aka Lens of Sutton did his National Service in the area and photographed many of the stations. They mostly look very neglected. From memory the last Jinties were moved elsewhere in 1957 and I presume the 57s took over although towards the end 08s were used to move traffic from Yniscedwyn.
The bridge over the Twrch at Gurnos was in a bad condition. John Mann the former western region bridge engineer said there were plans to replace the timbers with prestressed concrete but this was never done. I suspect a 56 would have been too heavy. The track on the Swansea Vale was well maintained and so there would be no problem there. There was a railtour in the 1950s when a Jinty was clocked doing over 50 and it was reported that the riding quality was good.
If you want information on how the track layout changed over time, Cooke's books, available from Lightmoor are invaluable.