Apparently this has already been kind of tested (underground too!)

I took a tour of the Wieliczka salt mine when I was in Poland 10 or 15 years ago. As we started to enter the actual mine, the tour guide said something like :

The roof is salt, the floor is salt, the walls are salt, everything is salt. Lick the wall if you want, it's salt."

pauses for a few seconds while somebody licks the wall

Now I should inform you that I tell that to every single group I take through here and every single time at least one person licks the walls.

I think, ultimately, it depends on how often the thing gets used.

If it goes long enough between licks, the salt will dehydrate and kill any microbes on the surface. But that takes time. If two people lick it in very quick succession, germs could be passed from one to the other.

I wonder If the subway climate would add flavor. Or the person who was licking it before....

I licked a pole on a subway car in NYC and it gave me superpowers. Unfortunately they only lasted for 2 hours. Almost died falling out of the sky. Thankfully I was over Iowa and managed to land in a pile of hay.

I appreciate the pushback on this, as a "salt lick" on the subway is unhygienic and spreads germs easily.