This is a silly service pattern. Cathays is Wales's 6th busiest station and the busiest station on the entire suburban network north of Queen Street. Cathays is overwhelmingly a south-facing station – the suburban network should be called the Vale of Glamorgan Lines. Very few Cathays passengers want Cardiff Bay. In general, they want Central, interchanges at Central, or stations Cogan to Llantwit Major/Barry Island. If we're messing with the Bay shuttle, it should go to Caerphilly/Coryton as the less busy branches.
It's going to be hard to please everyone, especially when you have different train types coming on the electric and none-electric bits. Looking at Cathays as we were doing a couple of days back -
http://www.firstgreatwestern.info/flows.html?stn=3820 - one of the dramatic things there's is how much the pattern has changed in the last five years. And I would ask how much is the pattern effected by the routes offered - if there were (for impractical example) Cathays to Bargoed direct services, would that flow come up the rankings too?