Deep in the tunnel underneath Shea Stadium next to the New York Mets' locker room, an old cop sits and listens to the radio every night. As the Mets troop out to go home, they ask, "What's the score?"

It isn't the first-place St. Louis Cardinals they're inquiring about, although that score interests them plenty, too. They know the policeman is a New Yorker down to his arches. That means he's listening to the Yankees.

"Ahead, 3-2, in the eighth," he mutters, not even saying who's playing.

"A subway World Series, now that would be something," says taxi driver Edwin Halliburton, who played on the Boys High football team with former batting champ Tommy Davis. "Haven't seen that since I was growin' up.

"All people talk about in the cab is the Mets and Yanks. When they were both so hot, movin' up to second place, everybody got excited. And this week, with both of them losing (three in row), everybody's worried."

Maybe baseball never had anything quite like the Series rivalries between the Giants and Yankees (six times) and the Dodgers and Yankees (seven). In just 36 years, New York monopolized the World Series 13 times. A Subway Series wasn't a fluke, but a staple of the game.

Since the Mets were born in 962, this city has awaited a Bronx-Queens showdown. The pot's bubbling at last. The Yankees and Mets have been ignited by similar heroics, and have a similar central worry now.

In baseball, many can march, but few can lead. Earlier this year, the Yankees couldn't pitch and the Mets couldn't hit. Then Ron Guidry and Gary Carter went over the top and all sorts of leathernecks fell in behind them.

Louisiana Lightning, 10-11 in 1984, had won a dozen straight games. More than his 13-3 mark, Guidry's eight complete games (as many as the rest of the staff combined) have lent order.

Like Tom Seaver in Chicago, Guidry has made the late-career adjustment from power pitcher to all-arounder. Guidry, whose .690 career winning percentage is the highest in modern history, still can reach back for his fast ball and hard slider. But inning after inning, he's learned to alter speeds on both those pitches, and he's added a changeup.

Although he strikes out only half as many as in his 25-3 days, he now walks less than half as many (22 in 162 innings). Most heat kings, such as Nolan Ryan, stay with the smoke until it's gone. The greatest, including Guidry, Seaver and Steve Carlton, take stock somewhere in their 30s and add new inventory.

Yankee gentlemen such as Phil Niekro (9-8) and Ed Whitson (5-6) are overmatched if asked to be aces, but playing second and third banana to Guidry suits them fine.

Though the Yankees fell to fifth place before Guidry's turnaround, the Mets suffered far longer before finding their platoon leader. They never fell to fifth, but they were plagued by a hitting slump characterized by Keith Hernandez' seven-for-65 drought.

But then on the Fourth of July, the Mets got 28 hits and won a 19-inning marathon in Atlanta, 16-13. They've won 16 of 21 since and are 4 games behind the Cardinals. That night, Hernandez hit for the cycle and Carter, who caught until 4 a.m., had five hits.

"We're finally back on the upslope of the sine curve," says Manager Dave Johnson, the college math major with his tongue only half in cheek. "For 73 games, we averaged 3.4 runs a game. Then, for the next 17, we scored 7.0. That got us back to 4.0 for the season.

"If we average 4.2 for the year, we'll win the pennant."

As though the gods were tweaking him for being a stataholic, the Mets, after scoring 16 runs Saturday and 15 Sunday, this week scored one, zero and two runs as the Reds swept three games in Shea. Still, the Mets figure the bats are back to stay.

"The big 'starter' for us is usually Darryl Strawberry (who had seven RBI in one game). When he starts beating on the drums, everybody wants to join the music," says Johnson of his right fielder who missed six weeks with a broken hand. "Now he's back. We're all hitting and there's payback. What goes around comes around."

"It's only justice that we all broke out together," says Hernandez, grinning.

Just as the Yankees hope that Guidry holds his form (and the staff) together, so the Mets worry each day whether gimpy catcher Carter, who's hit .314 for his last 38 games, can stay upright. Should he need surgery on his right knee, which one doctor already has recommended, that reverse march could happen again as it did when Strawberry was on the disabled list.

"Before I get out of bed, I ask the knee how it feels today," Carter told Mike Lupica of the New York Daily News. "I ask it if it's gonna get me through the day. Then I ask it if I'm gonna be able to play. If the knee says okay, that's all the conversation I need."

All baseball-loving New Yorkers say a prayer for Carter's cartilage, just as they do for Billy Martin's cranium.

Since Martin replaced Yogi Berra as manager, the Yankees have gone 46-31 and have cut the Toronto Blue Jays' lead to six games. Once more, the words "Billy has really changed" can be heard.

Martin's old friend, Mickey Mantle, who has become a Yankees Cable TV color commentator, might be the definitive source.

"Hell, the last thing Yogi needs is a job and all that aggravation. Yogi's got as much money as (owner George) Steinbrenner," says Mantle.

"But Billy's different. Billy was dying. He looked terrible. When I saw him, I was worried. He needs the game. It's his life. I think he felt like he was never gonna get back in the game again. Like the bad boy who never thinks he's really going to get expelled from school. Well, Billy finally believed he was out for good . . . Maybe he's gotten the message and this time it'll be different."

Although it's true that New York has been waiting for its Subway Series since 1962, Los Angeles has been waiting even longer for a Freeway Series. The Angels were created in 1961, yet never have met the Dodgers in a Series.

Guess who's in first place these days? The Dodgers and Angels.

Now, if the Mets and Yankees get hot again, baseball could have New York vs. Los Angeles matchups in both playoffs with a double chance for a cross-town Series.

That's not just good news for fans in those big burgs. Are baseball owners ready to endure a strike with such once-in-a-generation megamarket potential on the horizon?