Currently, the route between London Paddington & Reading on TSW. Now, I'm started to proposal west of Reading into Cardiff Central (About 90 minutes with 13 stations west of Reading). Stations include: Newport (South Wales), Bristol Parkway, Swindon (Wilts) & more. Rolling stock: Classes 800 IET, 387 & 166. Freight services as well. https://wikipedia.org/wiki/Great_Western_Railway_(train_operating_company) https://wikipedia.org/wiki/British_Rail_Class_387 https://wikipedia.org/wiki/British_Rail_Class_800
Would have to be set in the BR era due to licensing. From Reading to Cardiff may also be too long in TSW terms, probably Bristol to Cardiff (or Swansea) may be manageable.
