Thanks to L for her vivid reply:

'When you flush the loo on a train the poop, pee etc (depends on what you're
indulging in...) just drops straight out of the bottom. It's not nice, I
saw about 20 not so pleasent deposits come tumbling from a carriage when I
was trying to catch the train back from Glastonbury. If you've ever seen a
festival loo you'll know what I mean, If you haven't, you're one lucky
person. Personally I think the sign would be better if it told you not to
flush while standing at a station, or while passing through on at extremely
high speeds. It could get messy... '

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But does this mean that track maintenance workers on the railway lines are up to their knees in human excreta?