Morally, you should pay.
Legally, not sure where you (or they) stand.
There is a common law tort of 'you caused the problem, you pay to sort it out'.
Alternatively, accidents do happen. Given the quality / importance of the garments, should they have insured them for accidental damage? If the use of the clothes is sometime later in the spring, cannot they be cleaned prior to that? Were you negligent, dancing around the carriage with an open vessel, or carefully nursing the cup on the table when the train jolted over some poorly maintained pointwork - see it's the railways fault! Was the luggage stowed safely in an overhead rack, or illegally placed on the floor / adjoining seat?
Personally, if I was on the first day of a holiday, and my clean clothes are now coffee coloured and smelling, and will remain that way all week, I would not be at all happy - as I do not carry excessive amounts of spare clothing 'just in case'. But if I was on my way home, perhaps I'm not as bothered, as I'll be washing everything else soon anyway.
I once sat in a seat on a train where someone had wet themselves, or rather the seat,

- now that's something you don't want to be walking around looking like and smelling of through no fault of your own. Fortunately it was on the way back from a holiday and quite close to my destination. embarrassing all the same. You can feel peoples eyes following you and commenting.
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It was over several items of pale coloured/cream-white sports wear being taken for an event later in the spring, including some pure wool cricket sweaters.
Cricketers clothes get filthy over the course of a game - think where the bowler polishes the ball on his trousers, or the fielder does a diving catch. Surely those clothes get washed and re-worn rather than new every match?