On 16 July 2012 the UK Government announced plans to extend the electrification of the network at a cost of £350 million. This was at the same time of the announcement of electrification of the South Wales Main Line from Cardiff to Swansea. This would also see investment in new trains and continued improvements to stations. It is thought to start between 2014 and 2019.
Generally some trains run from one line to another, joining at Cardiff Central eliminating the need for changing trains there. However they may not run for the whole length of the line. Services run between:
Bridgend/Barry Island and Merthyr Tydfil/Aberdare - incorporating the Vale of Glamorgan and Merthyr Lines
Penarth and Treherbert - incorporating the Vale of Glamorgan and Rhondda Lines
Radyr and Coryton - incorporating the City and Coryton Lines
^Cardiff Central and Queen Street are where all services pass through and are interchanges to the national network. Pontypridd and Barry are the two stations with the largest passenger numbers outside of Cardiff and they are only served by this network. See respective Wikipedia pages.