One aspect we've explored, particularly with Brian's excellent posts, is the way that Hymeks tended to work in west Wales, and Cl 37's in East Wales in the last couple of years before the Hymek cull.
Was there any particular rationale behind the usage of Cl. 47s or Cl.52s on freight work in South Wales, beyond the fact that Cl.47s were more likely to be selected for inter-regional freight? In Shrewsbury for instance, in 1970-72 virtually all WR freight coming through was Cl.47 hauled, Waterston-Albion being one of the obvious exceptions.