Mysteries of the Universe I

Why do the authorities who run the railways always put non-drinkable water in the taps on trains? Surely they must have to go to an awful lot of trouble to find any these days in Western Europe. Rather than tiptoeing about at midnight with a bucket filled from rivers and muddy ponds, wouldn't it be easier just to top up the carriage tanks with water from the city reservoir? Sure, they'd have to take the 'aqua non potabile' signs down, but, once this small effort had been made, time and energy would be saved for years to come.