I'm guessing you mean the 769's. Electric trains that used to work in London, that have have had diesel generators fitted. They are meant to be a stopgap until the new trains arrive sometime over the next couple of years. They make up a very small percentage of tfw's stock, and will only be seen on the Rhymney to Penarth route. Won't see a huge amount out because they haven't worked very well.
Sorry to burst your bubble, but converted trams from somewhere in Europe are not running the rails of cardiff or the rest of Wales.
The trains on the rest of the valleys network are either 150's or single car 153's, british built and trains since the day they were born 🤣