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booking a trip for later this year, and I've had a little 'lightbulb moment' as to why the carriage letters on the 800s don't go in alphabetical sequence: it's so that regardless of whether you're in a five, nine or ten-carriage train, your seat reservation will more or less be the same.
A, B and C will be the same whatever you're in. G will always be the cafe car. D and K will always be the composite First and Standard coach, whereas L will always be an all-First coach. it's not much, but I thought it was rather clever.