He was sitting on a granite bench at the McPherson Square Metro station -- a young man with a backpack, a baseball cap and a thirst. He was carrying a cardboard cup of coffee. He took a swig.

In seconds, two plainclothes Metro police officers swooped down on him. They identified themselves and explained to the man how he had broken the law.

He was very apologetic. More to the point, he was from Texas. Because he doesn't ride the system regularly and genuinely didn't know Metro's rules, he got off with a written warning.

About 20 minutes later, a woman at L'Enfant Plaza wasn't so lucky.

Metro officers Tracie Saunders and Jamal Anderson didn't see the woman drink from a cardboard cup of tea. But they did see her leave the cup on a metal shelf below a pay phone.

A clear case of littering. The woman, who lives in the Washington area, apologized and admitted that she had done it. "She said she had a lot on her mind," Officer Saunders reported later. The woman was given a $10 ticket.

Even if you're a regular Metrorail rider, you might never have seen police officers enforce "quality of life" rules. You might believe, as thousands do, that Metro doesn't really care if the subway system is overrun by illegal eaters and drinkers, despite rules that make all eating and drinking illegal.

But since December, Metro has assigned approximately two dozen of its police officers to the RATS squad. I followed three of them for part of a shift last Monday.

RATS stands for Rail Anti-Crime Target Shift. The RATS squad is Metro's first concerted attempt to tackle a problem that has been growing in recent years -- and has reached serious proportions in the past five.

RATS members, many in civilian clothes, ride the system at all hours. Their intent is not to arrest more people. The idea is to produce a cleaner and safer subway system.

"People don't realize that's why we do this," said Officer Kenny Bauer, another plainclothes squad member whom I shadowed last Monday. "But that's why."

Another reason: to keep Metro patrons from feeling as if they have to act as rule interpreters or vigilantes.

The night before I followed the three officers, one passenger had approached another on the platform at the Fort Totten Station. Passenger One told Passenger Two that smoking on the platform was illegal. Passenger Two responded with a right uppercut to Passenger One's jaw.

Luckily, no one was injured, and luckily, a police officer was at Fort Totten at the time. Passenger Two was arrested.

This typist has often recommended asking a kiosk attendant or a train driver to summon police if laws are being broken. This story proves the case. "I would never suggest people go up and tell them" to stop illegal behavior, Officer Bauer said. "You never know what the attitude of the person is going to be."

Nor is it 100 percent clear what Metro's no-eating and no-drinking laws mean and whether the laws will be enforced uniformly.

For example, can you carry a cup of coffee and not drink from it? Can you carry your lunch in a sack as long as you don't munch anything? Is consuming water different from consuming coffee, soda or alcohol? Does it matter if you're a regular rider or a tourist?

Officer Bauer said he will allow passengers to carry a cup of coffee as long as the cup has a lid on it and as long as they don't sip. "The law says you can't drink it. But it doesn't say you can't carry it," he said.

Note to passengers: Don't press your luck by telling Officer Bauer that you are such a careful sipper that you can drink your coffee without spilling it. A, you may be wrong. B, see above about sipping being against the law, whether you spill in the process or not.

Carrying lunch? Fine with Metro. Sipping water? Not fine. Same reason as above. Regular rider? You're going to be held to a much higher standard than you would be if you were visiting.

And please don't argue that it's a 95-degree August day, you're feeling dehydrated and the no-drinking rules shouldn't apply to water. As Officer Saunders put it, you were free to drink all the water in the world "before you came into the system." Besides, spilled water can lead to slips, which can lead to injuries and lawsuits.

Although RATS officers can and do go anywhere in the system, they focus on the busiest stations. So transfer points (Metro Center, Gallery Place-Chinatown, L'Enfant Plaza, Rosslyn) get an extra-hard look. And because RATS officers also are charged with reducing fare evasion and pickpocketing, they also tilt toward busy downtown stations (the Farraguts, Foggy Bottom-GWU, Union Station).

It's too early to measure the effectiveness of RATS. Eating, drinking and littering are still obvious and common throughout the system. Arrest figures for January are not yet available. And some members of the public seem to think that they can sip and sup even when a uniformed officer is present.

As the three plainclothes RATS squadders lurked in the shadows of the Pentagon City Station last Monday (they were hoping to catch fare evaders in the act), a uniformed officer arrested a woman who was sipping from a large, fast-food soft drink cup.

He began to write her a ticket.

As he did so, she continued to sip.

Obviously, perfection remains a long way off. But to see Metro take a serious swing at the eating-drinking problem is a welcome start.