When we were living at a unnamed theological college in Llandaff in 2008 , we often used the Radyr to Coryton line to travel around Cardiff , (especially after our car had been stolen and wrecked) . Trains run twice an hour Mondays to Fridays, I dont think they ran on Sundays. Most of the time the service was not that well used. That was a shame as it is the fastest and most painless way to get into town, and then catching trains to Barry or the valleys.
I believe this line was reopened as a imitative between BR and the old South Glamorgan County Council.
It struck me that with the fact that the Swansea mainline not going to be electrified in the foreseeable future. Why can they not electrify the local lines. Surely this is something that could be worked with the cooperation of the WAG, Cardiff, Vale, and valleys councils.
They should integrate the bus and train services like they do in the US, rather like the RTD in Denver.
If this was properly planned out with some vision, then Cardiff could see a solution to its problem with congestion.
Just needs someone with vision!