Smoking, drinking and eating is not allowed
would mean that if you try to smoke, drink and eat all at the same time, as one activity (good luck with that!), then that activity is not allowed.
But smoking, drinking or eating (as separate activities) would be ok.
Smoking, drinking and eating are not allowed
means that neither smoking, nor drinking, nor eating, is allowed. Collectively they are not allowed.
So the subway authorities probably do intend the latter meaning, although I'm just guessing.
Let's do a little research:
Aha. Neither smoking nor eating nor drinking is allowed. All three of them are forbidden.
Littering is also forbidden (littering is one action - use 'is'), as are large belongings (the belongings themselves are plural - use 'are').
The possession of balloons is also forbidden (the balloons themselves are plural but 'possession' is a single action - use 'is').
It seems particularly harsh to completely forbid 'sitting'. Is this a mistranslation of 'sitting on the floor'? Or does this sign apply only to an area without seats (such as a corridor)?