Poking about on the internet indicates that my memory of them from that time needs clarifying slightly..!
1980 would be very much more likely for my Bristol to Cardiff experience of them...
According to the Vintage Carriages Trust online register, the ones that entered preservation did so in 1981... (although how guaranteed or secure is 'preservation' always..? I was aware of the ones at the Dean Forest Railway, wasn't aware that they don't survive, see post above. And the VCT register describes the Wensleydale pair(?) as "at risk"..)
Late in their BR career they could be found on Cardiff to Weymouth trains, and there's an album of pics of them on these online (sorry can't share link on this phone). These are descibed as summer Saturday services... maybe their regular year-round weekday use at this time was Bristol to Cardiff morning and afternoon peak services..?
These pics show that their non-standard experimental bi-fold doors had been replaced by standard conventional Mk1 doors during a mid-life rebuild (said elsewhere to be in 1971). So I'm not sure what my memory of the apparent difference was, apart from maybe the difference in alignments at solebar level. (This obvious difference was retained throughout their lives... Another difference was the windows.. slightly less deep, possibly slightly longer, and with different sliding vents than on standard Mk1s, prefiguring those adopted for the early Mk2s.)
There's also a short b&w British Pathé film from 1964 on YouTube (sorry can't post link) of the XP64 launch, called "Train of Tomorrow", with exterior and interior sequences, showing certain of the experimental features which were adopted for future builds of BR 'express passenger' stock (the Mk2 builds from 1965 onwards) and others which weren't..
Also a BR publicity photo from the NRM archive online, of the bright new modern image of D1733 and the Derby-built XP64 stock in the Derbyshire landscape, launching BR's new 'corporate image' incorporating an early interpretation of 'blue and grey' (described here as applied as 'turquoise blue and ivory' rather than what became standard from 1965/6 onwards) and double arrows logo on a red background (as widely adopted except for on rolling stock!) and yellow front warning panel (which I think was already being applied at this time, and was thus an incidental part of the new image, but incorporated nonetheless).
So XP64 for 'express passenger' and 'experimental', heralding new design features and a new 'corporate image' for BR, and in many ways a prototype for future builds of Mk2s (although the actual prototype Mk2 carriage was being built at Swindon works in the same year, and was turned out in standard maroon!).
All in all a key stage in the development of BR, both for passenger stock development and image/identity... Let's hope the survivors can be saved...
(I like them a lot, and the whole image that came with them, in part because I date from 1964 also
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