There was talk a couple of years ago about the line to Creigiau being rebuilt with a new station in Fairwater called "Keyston" being included, but canvassers for Plaid Cymru during the council elections told me that the new station would never happen and doubted the timescale for the line reopening.
There's a map on this link
http://www.bbc.co.uk/news/uk-wales-31418603
which includes the line to Creigau that in fact goes on to Talbot Green before joining the Swansea to Cardiff line at Pontyclun. There's also stations planned for Ely Mill, Culverhouse Cross and Rhoose and stations called Newport Road and St Mellons for east Cardiff, but it seems to me that a lot of existing stations (including Ninian Park) have disappeared!