When you're a child you tend to notice things more because your brain hasn't quite got used to taking things as read and using shortcuts.
One thing I always used to notice during childhood were the cryptic letters you'd see on the window frames (and occasionally in other places) inside London tube train carriages.
They looked like Letraset or decals of some kind, and the way that they were applied was always slightly wobbly manner. None of them were real words, but they still stuck in my head. TVIC. EPBIC. ComCOS.
No explanation, they were just there, mysterious runes or messages left by tube spirits for each other. EL. BRV. Sometimes they were accompanied by a star ★.
Of course nothing stays hidden for ever. These are known as Cant Rail Abbreviations as I discovered from a list of them online.
TVIC is Triple Valve Isolating Cock. EPBIC is Electro-pneumatic Brake Isolating Cock. ComCOS is Compressor Cut-Out Switch. EL is Escape Ladder. BRV is Brake Release Valve.
So now you know.