Ah ... it was the computer programmer in me thinking "what a logically silly thing to say". Of course trains which leave after midnight are in the new day ... that's when time goes back to zero after all! I guess I saw it along the same lines as
"this door to be kept shut at all times" [so why is it a door rather than a wall] and "this door is alarmed" ["who frightened it?"]. It was - you'll note - a "lighter side" post
In all seriousness though, Someone like me who has an ASD (Autistic Spectrum Disorder). You may read that sign and not pass through the door when they need to, because it says must be kept shut. (For those not in the know, people with ASD take things very literally!)
An example would be the Disabled Toilet on Platform 3 at
BRI» . Has the exact above sign... How are you ment to get into the toilet? Air vent anyone