I should probably know better than to ask this question, but how could you possibly know what % of toilet visitors only urinate?!
Sometimes more than just splash back is on the cards, certainly based on my years on the Networker fleet, and seeing the results of their toilets “reversing” when flushed, on a couple of occasions

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Generally the person concerned would appear on the platform looking disoriented, covered in mostly brown liquid matter, stinking to high heaven, and would be recommended to make a precautionary visit to hospital. The key takeaway is, when visiting the loo on a train, *always* lower the seat before you flush…