MTA Chairman Pat Foye this week highlighted a chief reason for his agency’s runaway overtime costs: Workers get too much time off. So what are the agency’s leaders going to do about it?

Foye dropped the news at Wednesday’s board meeting: The average MTA employee works just 40 weeks a year. That’s 12 weeks worth of vacations, sick time, holidays, etc. — year after year after year. Do you get that?

The resulting low “employee availability,” Foye explained, is a key “driver of overtime.”

Give your workers more time off, and you have no choice but to “backfill” by paying OT. That supposedly costs less than staffing up to cover the gap, because new hires would qualify for expensive fringe benefits.

Plus, of course, firing unionized public employees — even if you suddenly have too many — is a prolonged, expensive affair.

Over the last decade, “availability” has dipped steadily, and overtime has soared. As The Post has reported, extra-hour costs at the LIRR alone shot up 30% last year, to $225 million, pushing total MTA overtime outlays to $1.3 billion.

Yet employee time off, as Foye admits, is “largely the result of contractual provisions in collective-bargaining.” That is, the agency has agreed to union-contract demands for such generous “undertime” that it now finds itself hostage to outrageous overtime and the onerous costs that go with it.

The same contracts also make hiring extra workers — to fill in at lower, straight-time pay rates — so expensive.

Foye means to address the “undertime” issue in negotiations with the MTA unions over new contracts. Sounds good — but the list of costly MTA perks and work rules is long. Foye & Co. will have to work overtime themselves to fix this mess.