They fit the Rhymney line. Just north of Radyr is the restrictionThere are no PRM 153s. Currently no Rosco has announced plans to PRM them. There are issues with the vestibule behind the small cab and overall seating would be greatly reduced by the large toilet.
Also if run coupled to 150s it will stop them running on some of the Valleys as 153s are 23m long and don't fit.