Looking at the South Wales Valleys, with local knowledge, some lines stand out as lost opportunities in the Beeching era and afterwards. It seems that traditional working of some branches was firmly rooted in the ninteenth century and little thought had been given to the changing demographics following the Second World War and the rundown of heavy industry in the valleys. The Valleys are ideally suited to rail travel because they all run, roughly north - south, to the sea where the employment prospects are greatest
Aber Valley: Senghenydd - Caerphilly. The route served a short densely populated valley. The branch was only about three miles in length and the trains should have run through to Cardiff rather than terminate at Caerphilly, not only to avoid the necessity of changing but to provide additional capacity on the heavily used Caerphilly - Cardiff section.
Caerphilly - Newport to serve a growing area of population to the south east of the town and serve main line connections at Newport.
Porthcawl because, together with Nottage, it is a dormitory area for Cardiff and Swansea and, of course, a popular seaside resort. It would have benefited from through services to both cities. The layout, at Pyle, permitted this but was seldom exploited except for summer excursion traffic.
One only has to look at the lines which have been re-instated to see what should have been done. All the lines that served Cardiff, without a change of train survived the cull. All of those that have re-opened serve Cardiff direct: Maesteg; Aberdare; Ebbw Vale and The Vale of Glamorgan. Each one has proved to be a success. There is no reason to suppose that others with the same facility would not be equally successful.
There are other more marginal cases but I'll leave those alone for now.