NEW YORK -- Claudio Nisi stood on the 53rd Street subway platform as the F train screeched to a halt, the sliding doors revealing a subway car so full of human bodies that no one else could possibly get on. Nevertheless, more than a dozen people pushed their way in as he watched, frowning.

"I'm not getting on these things," said Nisi, a banker from Flushing, as he let five Queens-bound trains pass without boarding. "They're packed in like sardines. Ninety percent of the time I see people sticking out the door."

Riding the New York subways, particularly on certain lines at certain hours, is a test of the limits of human endurance. Elbows are jammed into abdomens, shoulders into backs. Crushed straphangers struggle for balance as the train lurches forward. At each stop, passengers must push and shove just to get off the train, while hundreds are surging to get on.

Between 8:45 and 9 a.m. each weekday, 12,700 people squeeze onto seven E and F trains from Queens to Manhattan. By 9:30, the line has carried 98,000 people to downtown office buildings in a little more than two hours.

The No. 4 and 5 trains on Manhattan's Lexington Avenue line, which stretches from Brooklyn to the Bronx, carry 10,300 uptown passengers in a 15-minute period each morning.

All told, more than 600,000 people ride these two lines every day, more than the 450,000 daily passengers on Washington's entire Metrorail system.

With 27 lines, 463 stations, 6,150 cars and 230 miles, this city's 84-year-old subway system is unlike any other. It carries 3.5 million passengers each weekday, about one-third more than the combined ridership of the next nine largest urban train systems, from No. 2 Chicago and No. 3 Washington to No. 10 Cleveland.

Cramming 250 or 260 people into 75-foot subway cars with 72 seats -- and repeating the exercise every two to three minutes -- takes a heavy toll in terms of delays, equipment breakdowns, safety and psychological stress.

Passengers frequently wedge themselves between doors, preventing trains from leaving. Or they are dragged along by departing trains -- 37 times this year -- as in a spate of recent incidents that included one fatality. This week, a young pregnant woman was pushed onto the tracks but survived unhurt.

Last month, a 27-year-old Queens woman on a packed Lexington Avenue train turned on a man she said was fondling her and pushed him out the door. She repeatedly punched him and banged his head against the wall until police intervened.

The New York City Transit Authority recently proposed a new density standard that would give each standing passenger three square feet of space -- enough so that "restricted movement can be made without touching another person," authority planner Michael B. Grovak said.

But the agency admits that even this modest level cannot be achieved on its most overcrowded lines, where passengers ride "cheek to cheek," as Grovak put it.

Critics scoff at the proposal. "When they calculate how many people can be crammed into a subway car, they don't take account of packages or umbrellas or elbows," said Gene Russianoff, head of the Straphangers Campaign, a grass-roots coalition.

The Transit Authority says its most heavily traveled lines are strained to their physical limit, but Russianoff says the real constraints on beefed-up service are budgetary. "The result is lousier, more crowded, less reliable service," he said.

The sheer number of people who pour onto the trains each day is a major cause of delays on the overcrowded Lexington Avenue, Broadway, Queens Boulevard and Flushing lines. The longer it takes passengers to get on and off at each station -- particularly on old, narrow platforms such as those at Grand Central Station -- the longer the "dwell time" at each stop. The line comes to resemble a congested highway, with backed-up traffic causing trains to slow to a crawl.

"The real crush loads come when people refuse to get on or can't get on," said Millard L. Seay, the Transit Authority's director of scheduling. "The people who can't get out of the station have to wait for the next train, and it builds."

Officials say they are scheduling rush-hour trains two minutes apart, as fast as the mechanical signal system will allow. But they concede that this is a theoretical goal.

"It just never materializes," Seay said. "You would have to have everything happening with split-second timing. Anything from a sick passenger to a breakdown to something thrown on the tracks messes you up."

A summary of 250 recent delays on one line included the following reasons: "refuse collector, stuck brakes, no power, smoke, sick passenger, switch trouble, missing glass, passenger assaulted."

On a recent Monday at 6 p.m., passengers bound for Queens from the 53rd Street station were forced to wait on the staircase. From the platform edge to the back wall, thousands of grumbling commuters stood five and six deep for the length of two football fields.

The mob scene was caused by two broken-down trains on the intersecting Broadway line, which in turn blocked all E trains from leaving the World Trade Center for 32 minutes. By the time the E trains began heading uptown, they were packed, and the crowds had to watch them go by in frustration.

The Transit Authority says it has as many as 225 such incidents each day for the entire system. A Straphangers Campaign survey found that 29 percent of medium-length train rides (20 to 45 minutes) were more than five minutes late, and one in 10 was 11 to 20 minutes late.

The authority is looking to boost service before and after the rush-hour peaks, but Grovak likened running 30 trains an hour to revving an automobile engine at top speed. "It's not something you can maintain indefinitely," he said.

Seay, who worked for Washington's Metro for 16 years before coming here in 1985, said any perceived overcrowding problems on computerized Metrorail trains are "minor league" compared to manually operated New York. Metro spokeswoman Marilyn Dicus did not quarrel with the description, noting that the system has the capacity to add longer and more frequent trains as ridership grows.

There is no such elbow room here, where trains are already as long as the platforms will allow. Rush hour has expanded, with seats filling up as early as 6:30 a.m. on some lines and remaining filled as late as 7:30 p.m. Many commuters have simply rearranged their lives, lingering for an after-work drink until the subway crowd thins out.

After a $7 billion modernization effort, service has improved on lines with new or overhauled cars. But as ridership has grown -- as much as 37 percent on the Lexington Avenue line -- overcrowding has worsened.

One result is that the subways have failed to lure many people out of their cars. Daily auto traffic into Manhattan has risen from 842,000 in 1972 to nearly 1.2 million in 1985, as many people apparently decided traffic-choked roads and $300-a-month garage fees are preferable to fighting the crowds underground.

Still, the subway system cuts across class lines like no other New York institution. There are stockbrokers in three-piece suits and teen-agers in leather jackets, smartly dressed women with packages and beggars bellowing for spare change. Many New Yorkers interviewed underground said with a shrug they take subway service in stride. Others were more vociferous.

"I hate it -- the smell, the fact that I don't know if I'm gonna get a knife stuck in me," said a Manhattan woman who has a short daily commute on the Lexington Avenue express. "I pray for my life for six minutes."

"They don't let you know if there's a delay until after you pay your dollar and you're down here," said Vita Mauro of Queens as she squeezed onto an F train.

One measure under consideration is to bring back platform controllers to keep riders from shoving and holding doors at the most crowded stations. Asked how any employe could control an unruly New York crowd, Seay said, "Other than carrying a cattle prod, I'm not sure."