Would you rather have x services a day, or x - a few to allow for time to clean the trains outside fully?
London Midland seem to be able to keep theirs clean by putting them through the washer once a day or thereabouts, I believe. I'd venture the view that Eurostar, and for that matter Northern, aren't doing this. They have mucky carpets, but that just highlights why carpet is a bad choice for regional rolling stock and vinyl would be better - all you need to do then is mop it out. It looks spartan, but you can use e.g. wood effect to make it look more homely.
OTOH muck on the windowsills is just sloppy. Though dont get me started on why no TOC cleaners bother cleaning the black filth off ventilation grilles - it takes a quick wipe with a cloth yet it's never done. Why I wonder?
As for inside, again a decent clean once a day, and litter pickers walking through. VT even do this during each journey.
Neil
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It's doesn't help that the interior is pretty drab and dark, my experience is that the trains are cleaner than they may actually appear. The outside is a different matter, but that's what happens when you paint your trains white
Actually, SWT (given your profile pic) are excellent at keeping their trains clean as well, including the white ones.
Not so doing just shows "can't be bothered". And TBH, the feeling of "can't be bothered" echoes throughout Eurostar's entire operation.
Neil