Now that both the Yankees and the Mets have won their first-round playoff series and advanced to their respective league championship series, New Yorkers are psyched even more. What they once considered impossible and later believed improbable is beginning to look almost inevitable to them. Another week and they believe the World Series will bring together the two local teams for the first subway series since 1956. Yankees fans are confident--smug. The signs out at Shea Stadium read: "Ya gotta believe."

However long it lasts, whether there's a subway series (or an "E-Z Pass" series for those using the bridges), no feeling of baseball excitement can quite match what's happening in New York. Even if the two teams are not playing each other (yet, they say), the Yankees and the Mets have never played each other in the postseason. Already, the city is divided as it used to be before the Dodgers and Giants were uprooted. Sister Francis Wystepek, Yankees fan, is a "Blessed Bomber"; her friend Sister Kathleen Maciej, a Mets fan, is known for her "amazin' " grace. Such is rival fandom in the city.

The subways already were filled as the Yankees destroyed their hexed foe Texas and the Mets reduced Randy "The Big Unit" Johnson and then obliterated Arizona with a 10th-inning home run by a reserve catcher. Yankees fans, bundled in clothing with Yankees insignias for 40-degree nights, crowded onto the No. 4 train from Manhattan to the Bronx. The No. 7 departed for Queens with guys wearing Mike Piazza and John Olerud uniform jerseys; on Friday, the train operator announced the most popular stop on the line in the form of a prediction: "Shea Stadium, home of the world champion Mets."

"It's great being here at Shea. Yankee Stadium is a dump." So said a man packed somewhere in one of Shea Stadium's finest freight elevators. This tomb of humanity creaked upward, in this ballpark stuck in the '60s, carrying lost guests from the less-than-extraordinary Diamond Club, sleep-deprived sportswriters and stadium employees. One manned an overfilled jumbo trash container and another a wobbly food cart, 10 feet tall with several shelves holding 100 cream pies. That was Friday. On Saturday, when the same elevator stuck between floors, one of the passengers, the redheaded former Met Rusty Staub, laughed and said to the longtime operator, "If there's one person who knows how to work this thing, it's you." Le Grand Orange was as cool as he used to be at the plate as everyone else sweated in the immovable box.

Ah, the door opened. Shea never looked so good. The crowd was thick with working men and women, some of whom talked of having saved their money to buy tickets. Yankee Stadium had the greater number of rich and famous. Wellington Mara, the owner of football's New York Giants, sought "Mr. Steinbrenner's box." Billy Crystal chatted up former Yankee Don Mattingly. In Yankee Stadium, even the public address announcer is legendary. Bob Sheppard, 49 years behind the mike, sounds as if he's intoning from the bottom of a well: "Ladies and gentleman, here are the lineups." Those mere words were greeted with a roar that could not have been louder if Babe Ruth brought out the lineup card.

Shea Stadium and the big ballpark in the Bronx are in different leagues in more ways than one. Monument Park, with its memorials to former Yankees greats, is a landmark, visited by fans who act reverentially, some speaking in whispers as they walk amid the markers. In the corresponding spot in left-center field at Shea is an extraordinarily humble bleacher section, tiny actually, looking as if it could be disassembled in five minutes.

Then there is the roar of planes at Shea. Despite takeoffs and landings at La Guardia, Shea's video board proudly presents, every game, "The Great Shea Airplane Race." Enough with planes; yet four "planes" with colors to match the stadium levels roar across the screen in a "race" above the city, with the first to clear Shea the winner.

Both stadiums play Sinatra records. Both teams have shortstops with magical gloves, Rey Ordonez and Derek Jeter, although the Yankees' Jeter hits better. At both places, fights in the stands occur regularly--New Yorkers take strong exceptions to New Yorkers' outspoken opinions. Both stadiums play "YMCA," but at Yankee Stadium the grounds crew dances to the music while dragging the infield in a routine that somehow doesn't grow old. One of the beauties in being at either place is avoiding the games on television, which features close-ups of managers looking anguished and spitting. In person, it is a pleasure to see the whole landscape.

The best sideshow at either stadium is Bobby Valentine, the Mets' manager, jousting with the baseball writers. Valentine is outspoken, which gets him in trouble. Sometimes he puts a spin on what he has said--like, it didn't come out in Sports Illustrated the way he meant it when he was reported to have said he was managing a bunch of dummies.

Actually, Valentine is so media savvy that on Friday he even supplied part of the punctuation to one of his own quotes. Speaking of the ailing Mike Piazza's availability, Valentine repeated a conversation he'd had with the catcher: "He said, 'I want to play but dot dot dot I don't think I can.' " Valentine did everything but supply the quotation marks.

I was sure that the Mets would be overmatched by Atlanta when the National League Championship Series starts Tuesday. But Valentine gave me second thoughts with an odd perspective, which brought memories of 1969, when the Mets couldn't possibly beat Baltimore, and '86, when they couldn't possibly beat Boston--until, in both instances, strange things happened.

"The next team we are playing is going to be playing, I guess, against some ghosts," Valentine said, "because they said we were dead. So I don't know if they have ever played against people who have come back from the grave."

This much is for sure: New York fans already are getting their subway tokens for another round of playoffs in the Bronx and Queens. The lordly Yankees remain the best team in baseball until proven otherwise, and the Mets ain't dead yet.