South Wales Coastal came out in 2017, and is still available in bundle form from places like Humble Bundle, with licensed trains. Those licenses have expired and the standalone version of the route – as well as the Swansea extension – is removed from Steam. The version that you would have purchased recently was granted for free to all owners of the existing Swansea expansion as it literally remakes all existing content from the pair of DLC, but removes the licensed trains.
This can have issues. For instance, the 150/1 scenarios will not show up on the newer version and may not even work. They won’t even work on what you can get from Humble. Those three scenarios are literally locked unless you picked up the Cardiff-Swansea extension before it went away in April 2020. Scenarios created for that version of the line would need to be remade for the newer line to operate. I’m sure not all have been. The fact that DTG didn’t recreate the 150/1 scenarios shows how much they care about that issue.