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Every year around this time, a silent menace threatens the region’s transit system.

Falling leaves.

Already this season, slippery rail conditions caused by leaves have delayed Metro-North and Long Island Rail Road trains more than 700 times.

On the subway, the B and Q trains have been held up regularly as they go aboveground through Brooklyn’s leafier precincts.

How can fluttering foliage bring down a mighty rail network?

The culprit is pectin, the same stuff that causes jelly to jell.

It’s in leaves.

When trains run over wet leaves, “it actually creates a slurry,” said John Kesich, a vice president at Metro-North.

Because trains on slippery rails are harder to slow down, the railroads dial down speed limits and order drivers to start braking earlier.

Hence the delays, which average 10 minutes.

Metro-North power washes most of its track each night. But the leaves return.

Even with careful braking, wheels slide on the leaves. This creates flat spots on the wheels.

Over the course of the year, just about every wheel on every Metro-North train has to go into the shop to get re-rounded.

Those are some powerful leaves.

Here’s what else you need to know for Tuesday.

WEATHER

Sunny, windy, chilly, with a high of 47. Down to freezing tonight.

COMMUTE

Subways: Click for latest status.

Rails: Click for L.I.R.R., Metro-North or New Jersey Transit status.

Roads: Click for traffic map or radio report on the 1s.

Alternate-side parking is in effect all week.

COMING UP TODAY

• Mayor Bloomberg signs nine bills, including the one raising the age for buying cigarettes to 21 and others requiring flood-proofing for buildings.

• Marian Wright Edelman of the Children’s Defense Fund speaks at John Jay College on raising the age of criminal responsibility in New York.

• Community Board 1 in Manhattan will hear a proposal to replace an old building at the old Fulton Fish Market with a high-rise.

• It’s “Tasting Tuesday” at the food stalls in Bryant Park’s Winter Village. 4:30 to 6 p.m. [Free]

• Bloomingdale’s unveils its holiday window display. 5:30 p.m. [Free]

• A “town hall” meeting on improving health care in Brooklyn, where four hospitals have closed or face closing, at Brooklyn Law School. 6 p.m. [Free]

• A lecture on “Unearthing Lost Histories of the Ancient World” by the historian Joan Breton Connelly, at N.Y.U. 5 p.m. [Free]

• For more events, see The New York Times Arts & Entertainment guide.

IN THE NEWS

• “Spider-Man: Turn Off the Dark” will close in January after three sharply mixed years. [New York Times]

• A Catholic high school in the Bronx postponed a lecture by a retired priest who says that gay people can “pray away” their sexuality. [New York Times]

• Plight of the New York City dirt biker: all revved up with no place to ride – legally, anyway. [New York Times]

• A 32-year-old man is sought in Sunday’s rape in Central Park. [Daily News]

• A 58-year-old who had served 24 years for a Brooklyn murder was acquitted at a retrial. [Associated Press]

• Chicago, with only one-third of New York City’s population, had more murders last year, the F.B.I. said. [Washington Post]

• Stop-and-frisk encounters are down 80 percent. [Newsday]

• The correction officer who had a son by a death-row inmate lost custody of the boy permanently. [Daily News]

• A Harvard professor has analyzed the Marquee nightclub’s success. Findings: They’ve been charging a lot for booze and “focusing on electronic dance music and star D.J.’s.” [Daily News]

• Ed Koch’s old dining-room chairs fetched $11,000 at auction. [NY1]

• New Jersey’s governor, Chris Christie, says New York City is headed in “the wrong direction” with Mayor-elect Bill de Blasio. He invites New Yorkers to move to his state. [New York Post]

• Coincidentally, a poll found New York State’s voters would prefer Mr. Christie to their own governor, Andrew M. Cuomo, in a presidential matchup. [Daily News]

• The Los Angeles Times looked at the phenomenon of New York City dwellers who live in R.V.’s.

• Shake Shack’s owners will open a restaurant in West Chelsea featuring “Southern-inspired bar food.” [Grubstreet]

• Scoreboard; Blazers beat Nets, 108-98.

AND FINALLY…



What to do with an unemployed cheerleader in chief?

Brooklyn’s borough president and biggest booster, Marty Markowitz, has become one of the city’s best-known politicians while wielding very little actual power.

But term limits have put him out of a job at the end of the year.

Yesterday, two fellow Brooklyn Democrats proposed broadening his portfolio: They want Mr. de Blasio to name Mr. Markowitz “ambassador for N.Y.C.”

Mr. Markowitz told Politicker that he found the notion “flattering.”

A spokeswoman for Mr. de Blasio said, “We have no comment on this.”

Joseph Burgess contributed reporting.

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