Coradia to Cardiff?. Anyway, the class 175 would be a nice addition to TSW2. I mean, Wales is beautiful. In my opinion, it's just as nice as Arosalinie. Plus the 175 is a beefy unit. Combine the 2 *bang* and you have cooked up a delicious route. As always thoughts are appreciated.
I did suggest a few weeks ago, part of the Marches Line maybe Newport to Hereford has the potential to be a crowd pleaser. Maybe that's what the OP is thinking of. The South Wales main Line itself, apart from around Pontyclun, is fairly dreary and flat - not really in the same league as the Arosa route, flawed as it is!