NEW YORK, NEW YORK

“Down and up!,” the policeman yelled at the crowds that swarmed onto the platforms at the Willets Point stop in Queens.

“Down and up!,” he shouted again. “Express train to Grand Central!”

Dutifully, New York’s baseball fans clattered down two flights of stairs and up two more. They found more than a dozen red and gray cars waiting on a track pointed toward Manhattan and, eventually, the Bronx.

When the doors rattled and closed and a tinny voice from above told them, “This is an express,” they cheered as if just told the subway crew would be serving dinner, and it was pot roast night.

That was how fans of the interleague series between the Mets and Yankees--and the first major-league doubleheader to be played in two different venues in nearly a century--changed ballparks early Saturday evening.

The players took buses. The umpires took limousines. The fans crammed into the subways that jerk and rattle in dank tunnels beneath their city. They stood shoulder-to-shoulder on the 7 train from Shea Stadium’s right-field line, changed to the 4, and emptied out just beyond the left-field foul pole at Yankee Stadium.

When the doors opened in the Bronx, the first voice they heard was legendary public-address announcer Bob Sheppard’s. It was a recording. Something about no bottles in the stadium, but it didn’t matter.

One look at the famous facade, and a square-jawed man in a navy Yankee shirt thrust his arms over his head and exclaimed, “I’m home!”

On one historic day, everybody was.

The Yankees beat the Mets twice, 4-2, before 54,165 at Shea Stadium, then 4-2 before 55,821 half a day later at Yankee Stadium.

On Sept. 7, 1903, a year before the New York subways opened, Brooklyn’s Superbas and New York’s Giants split two games, the first played at Washington Park in Brooklyn and the second at Polo Grounds in Manhattan.

The Superbas eventually became the Dodgers, both teams eventually moved west, and until the forces of Bud Selig’s interleague play and a June rainout combined, there would not be another dual-venue doubleheader.

So, 97 years later, a 20-ish guy in a Yankee jersey riding the 7 train considered the historical implications of all that and said, simply, “Knock, knock.”

A man in a Met jersey returned his gaze and said nothing.

“Knock, knock,” the guy repeated.

“Uh, who’s there?,” Met jersey said.

“Mikey’s oh-fer,” Yankee guy said.

Met jersey twisted his eyebrows around and bit.

“Mikey’s oh-fer who?,” he said warily.

“Mikey’s oh-fer the series! Ha-ha!”

Mike Piazza, the Met catcher, was hitless in eight at-bats in the first two games of the interleague series.

“Stupid,” the guy in the Met jersey said under his breath.

In a place that smelled like the bottom of a closet, it passed for humor.

Considering the usual relationship between Met and Yankee fans, it was almost tender.

The subway is many things, but on this afternoon, it is Met fans and Yankee fans mingling.

It’s “Let’s Go Mets” vs. “Let’s Go Yankees,” most of it at remarkably high decibels, Queens vs. the Bronx, Piazza vs. Jeter, Mr. Met vs. the Boss.

It is the occasional crack about Wayne Tolleson.

It is also the town that never forgets.

On the crowded 42nd Street platform, five Met fans chanted loudly for several minutes as everyone waited for the 4 train.

A train did eventually arrive, and the five loudmouths shoved their way on.

“Long day for Met fans,” an older gentleman said to his wife as the train pulled away.

“Think they’re going to lose twice?” she asked.

“Oh, I don’t know,” he said. “But they just got on the 5 train.”

While there are small lines of allegiance drawn all over town, the thickest lines were smeared across the world’s most famous skyline.

Tino Martinez, the Yankee first baseman, hits a home run at Shea Stadium, and the fans there cheer wildly.

Piazza is hit in the head by a Roger Clemens fastball at Yankee Stadium, and the fans there boo.

Met Manager Bobby Valentine is ejected from the first game for arguing an obstruction call after exactly one pitch.

Well, everyone cheered that.

In the middle of a town whose residents were absolutely determined to choose sides, however, there strode a pitcher who once represented everything the Mets were, when they ruled New York and the Yankees were nothing but a pitiful, Steinbrenner sideshow.

Dwight Gooden, their Doc, pitched against them.

Only the night before, Gooden, not yet on the Yankee roster, drove past Shea on the Grand Central Parkway. He was headed to dinner. The stadium was lit up, as the Mets and Yankees played the first game of the series. He took his eye off the road for a moment and peered inside the ballpark. He saw the crowd.

“Right there, I got goose bumps,” he said. “The last couple of days have been more than I could imagine.”

At about the same time, David Rodriguez had forced his yellow cab along the jammed Van Wyck Expressway, which also passed near Shea.

He shouted over the front seat, “They’re going to have to put more trains on those lines.”

He grinned and nodded.

“They all want to see Doc,” he said.

Billy Connors is George Steinbrenner’s pitching guru. He is a big, round guy who lives in Tampa Fla., so he can be close to the Boss and the pitching prospects and rehabilitation projects that come through the Yankees’ minor-league complex there.

Dressed in a lime-colored silk shirt and black slacks, Connors was in New York because Gooden was in New York. Doc was Connors’ project for the last month. Together, they arrived at Gooden’s new pitch, a screwball designed to break down and away against left-handed hitters.

“He didn’t really have any off-speed pitches,” Connors said. “This is a freak pitch he can get over. We knew he needed something. No. 1, he needed to command his pitches. We tightened up the slider, worked on the sinker.”

All tuned up, Doc stood on that familiar mound for the first time since 1994, and for the very first time as an opponent, and let a standing ovation roll over him as he crossed the third-base foul line in the bottom of the first inning.

In five innings he gave up two runs, the second on a soft fifth-inning double by Derek Bell, who wears Gooden’s old No. 16.

“The young guys, they’re looking for anything,” Connors said, laughing quietly.

“Doc, he was looking for the one thing that might keep him going.”

Asked if he thought that, at 35, this was Gooden’s last chance, Connors said, “I don’t know. You gotta ask him that.”

Gooden shrugged.

“It’s just overwhelming,” he said.

The day had grown from almost nothing.

If New York really doesn’t sleep, then that would explain Saturday morning. The sun had been up for hours and there were only two people on the double-decker sightseeing bus rolling past Grand Central Station.

There were few cars and fewer people, only some shop owners hoisting the security gates in front of their stores, and a handful of poor souls rattling around beneath yesterday’s newspapers.

New York didn’t wake up. It came to.

Maybe it was the price of the night before, of Paul O’Neill’s catch, the delirium of the Yankees back in first place, however slightly, and of much more baseball to be played.

Even if New York was to host a nearly unique day of baseball, no one was pacing himself.

So when the sun oozed through the marred plastic windows of the early 7 train, it showed that Shea Stadium vendors were the car’s only inhabitants.

Brunch at Shea, followed by a late supper at Yankee Stadium, was late in starting. The stadiums and the subways eventually did fill, and every 10th guy looked exactly like Jerry Seinfeld.

They threw the first pitch in Queens at 1:21 p.m., EDT, the last in the Bronx at 10:44 p.m. In between, they made some history. They do that every century or so.