I've read through the article published in the Western Mail. Network Rail's press release yesterday was probably prompted by the end of the consultation period on the alterations to Central and Queen St stations. I'd had a look at them some weeks ago. They needed 'prior permission approval' (I think) for the building alterations and extention of the retaining wall at Central for platform 8. Track alterations, and the new bay platforms at Queen St were mentioned as not needing permission.
Little is new (to me), but some dots are joined up. The longer trains are coming from having electrified units. The Western Mail quotes 6 car trains as being the new norm (remember the platform lengthening?), and each car being 10ft longer than the Sprinters. Are there any likely cascades that fit this bill around 2017, or does this sound like a new build?