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The MTA is asking for $4 billion in federal funds to weather the coronavirus outbreak, which has severely reduced its ridership.

Writing to New York’s congressional reps, MTA Chairman Pat Foye said the agency needs the cash injection to keep public transit open while many New Yorkers stay home to slow the spread of the disease.

“The stark reality is that as more people stay home following the advice of medical experts, the M.T.A. is now facing financial calamity,” Foye said in the letter, dated Tuesday.

Subway ridership was down 60 percent on Monday compared to the same time last year, Foye said — while the commuter rails saw even steeper drops of 90 percent for Metro-North and 67 percent for the Long Island Rail Road.

Fares and tolls make up a huge chunk of the agency’s operating budget, and Foye said the MTA could be $3.7 billion in the red if the ridership slump continues for six months.

The agency also wants an additional $300 million for its coronavirus response, which includes a more frequent cleaning regiment, Foye said.

On Friday, the Feds authorized transit agencies to use federal money previously allocated to long-term maintenance.

But Foye said doing so would be a “huge blow” to the MTA, which has promised to spend over $50 billion on much-needed modernization efforts over the next five years.

He said another $6 billion in state and local tax revenue could also be in question.

“The nation will need a strong New York to fully rebound from the current crisis,” Foye wrote. “The mass transit system operated by the MTA is essential to that objective, and restoring it is a matter of national interest.”