The new rule applies to subways, buses and all LIRR and Metro North trains.
For months now, health experts have been saying that wearing a mask is an effective way to slow the spread of COVID-19.
Masks have also been mandatory on MTA trains and buses for months.
But violators now face fines, after Gov. Cuomo directed the agency to come up with an enforcement mechanism.
The goal is to keep the transmission rate low as the economy slowly gets back on track.
Interim MTA President Sarah Feinberg says mask compliance is already very high, but as trains get more crowded, covering your nose and mouth is that much more important.
"We're going to do everything we possibly can to get as many people wearing masks as possible," she said. "We know that masks stop the spread of the disease. We know that they keep our workers safe. We know that they keep our customers safe."
MTA ridership is down 70%, and the agency is bleeding money. Officials say this is one way to hopefully make more riders feel safe.
The MTA says forgetting or not having a mask is no excuse. In addition to PPE vending machines now located in some stations, the agency says workers can give you a free mask if you need one.
As of early Monday evening, there were not any summonses issued for mask non-compliance.
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