The MTA employee who told you no doesn't seem to understand that THEY too will be inhaling that smoke b/c it is all recycled air. There are a lot of junkies on the trains who will smoke far more than weed - I have seen people light up crude pipes made out of a bic pen. It smells like a combination of burning plastic and a strong industrial chemical odor. My guess is that is not weed but something far harsher like meth or fentanyl.

Edit - in fact, the MTA employees will actually be in a far worse position than the commuters long term b/c we at least have a destination to get off while they are inside the same train for hours at a time while the recycled air from different passengers is turned over and over again.