I can say one of the main reasons why local parents in the suburbs of Cardiff send their kids to Welsh language schools, Cardiff is generally not a Welsh speaking area of Wales. Is for a better overall education and avoid the poorer educational standards of multi ethnic schools in parts of the capital. Less about the benefits of being taught bilingually and more about being with other kids who's parents also have above average incomes.
The same thing happens in some non Welsh language schools that have very good results, e.g. Stanwell in Penarth, house prices in the catchment area are among the highest in Wales, the result being you tend to have very few kids from ethnic backgrounds as their parents are priced out. The long term result is higher house prices, result in better education, result in even higher house prices.
Back to trains, could someone please tell me why on earth the English 'Onboard' announcements on the new 231/756 trains are read by someone with a very pronounced North Wales Accents? If you are going to have a regional accent why not use someone with a Valleys or Cardiff accent? I suspect the reason is most of the communications roles in TfW are mandated to be bi-lingual and hence most of the staff are from North Wales.