Thanks to you and Llanishen Blue for your replies. The timetables may have changed during the pandemic, but I remember it was a bugger before because the train to Ninian Park from Radyr was always leaving just as my train arrived, so it was always a half an hour wait for the next one. That’s why I’d stay on the Treherbert train until Queen Street where I’d have a short wait for a City Line train going in the opposite direction, hence my question about whether the detour to the Bay applies only to City Line trains.