You say that like heritage railways, when they started, had a choice; "Dear BR, please may we have some carriages" "Certainly, what sort would you like?". The fact of the matter is that Mk1s were cheap and plentiful at the time many railways were starting up. Unless you were a railway that started before the mid-1970s, there really wasn't much left that wasn't chicken sheds. And they cost an absolute bomb to restore compared to your average dilapidated Mk1. A well restored Mk1 with original style upholstery, varnished wood veneers, some pretty paintings in the picture frames and string luggage racks can be virtually indistinguishable from the final designs of any of the big four railway companies. So whilst I completely agree that pre-1948 stuff is lovely and interesting, think a little more about those of us who slave away over Mk1s trying to make them as nice as possible because we don't have a choice, and because we care.