Wonder if Cross Country will run their usual 2 or 3 coach turbostars (170's) on the Nottingham > Cardiff route? They can't cope with normal flows.
If using National Express, note that they now arrive & depart from Sophia Gardens which is north of the Principality (Millennium) Stadium. Go to first bride N of stadium & walk up river on the west bank & you will find them.
It's not hard to find out. I provided the link to the National Rail page about RWC arrengments at the top of this page, where it says:
Millennium Stadium (Cardiff)
CrossCountry will run additional services between Birmingham New Street / Cheltenham Spa and Cardiff Central on match days.
First Great Western will run additional services between London Paddington / Bristol Temple Meads / Swansea and Cardiff Central.
A quick search in a journey planner shows that this coming Saturday a couple of the Manchester-Bristol Voyagers will be diverted to Cardiff to back up the 170s, giving 0930, 0942, 1030, 1042 and 1130 trains from Birmingham to Cardiff, with extras in the return direction as well after the Ireland v Canada game giving 1645, 1717, 1745,1820 and 1845 departures.
It may just be that they expect the morning trains to be thronged with Tongans and Georgians living in Birmingham off to Kingsholm for the noon kick-off, but I doubt it. They do seem to try to provide three-car 170s on match days at the relevant times, so it is to be hoped they diagram five-car 221s on the Manchesters as well.