Cardiff has relatively good rail links to the rest of the UK. There are direct services to London Paddington, Bristol, Bath, Birmingham, Manchester and Swansea. There is also an extensive service to the south Wales valleys, thanks to the coal mining history of the area.
When you arrive in Cardiff you will find yourself at Cardiff Central station - details below. Journey time from London Paddington to Cardiff Central is two hours and ten minutes, from Bristol Temple Meads to Cardiff Central is 50 minutes, both are regularly subject to delays and cancellation. In the next few years the railways around Cardiff are undergoing a massive investment programme which includes electrification of the lines, more platforms at the main stations and new trains (although these will more than likely be refurbished trains from London).
There are several local railway stations in Cardiff including Cardiff Queen Street, Cardiff Bay and Cathays for Cardiff University.
The main points that lets the rail service in the area down are the cost of rail fares, that there is no regional integrated travel card and the lack of any local stations on the mainline through Cardiff.