This is the sight which greeted passengers using one of the new trains en route from King's Cross to Welwyn Garden City.
Vandals had targeted one of the toilet compartments and sprayed it floor to ceiling in graffiti.
It means huge inconvenience and cost to train operator Great Northern to clean it up and return it to a state suitable for use.
Thameslink and Great Northern customer services director Jenny Saunders said: "This is mindless vandalism. It is ugly and makes people feel less secure, which is why we make every effort to remove graffiti as soon as possible.
"A single toilet like this will cost thousands of pounds to clean. We've just started a programme now to respray places where graffiti has been polished out."
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