"You looking for graffiti?" Mildred Nieves asks, her mouth slowly forming a broad, puckish grin. "What kind of graffiti do you want? Clean words? Dirty words? Names? Pictures?"

Then, moving swiftly through the sleek, silver subway trains, the Metro cleaning woman leads a rare guided tour of Metro graffiti.

In one car, the revelation that "Riff" and "Willy N.E." rode here is written in huge, chunky black letters across a row of windows. A love pledge, "Karen and Lee, June lovers," trails across a seat cushion in black spidery scrawl.

Most of the messages are brief. "Ass!" boldly spans an orange cushion. "Graffiti!" rides on an arm rest.

By the morning the graffiti gallery will be gone. Nieves and others will work through the night to erase the words and create the illusion that the still sparkling Metro system is untouched by graffiti.

Without that illusion, "some people would see the graffiti and think they're only one step away from being raped," says Metro spokesman Cody Pfanstiehl. So Metro is waging a war against Washington's graffiti writers.

At stake are Metro's desire to maintain ridership levels -- now 300,000 a day -- and the upkeep of the system's 250 railroad cars and 38 stations. If the trains become a multicolored, moving college like those in New York's subway system, ridership would surely decrease, Metro officials believe.

Among the first to go would be some suburban commuters, elderly people and tourists, they say.

As a result, when graffiti artists have attacked train station walls, Metro's 160 custodians have "had a mandate from day one to wipe it out as soon it occurred," according to Al Williamson, associate director of Metro's consumer affairs office.

By using heavy surveillance and promptly cleaning up the graffiti that is written, Metro has managed to hold graffiti on the cars and subway station walls to a minimum, according to Metro police chief Angus McLean.

"You do have some graffiti," McLean says. "Our policy is to get rid of it as soon as possible." Or discourage people from writing it in the first place.

The subway stations were designed with highly visible mezzanines and corrugated walls that are impossible to vandalize without being seen, he said.

Every station is monitored by at least eight cameras and a corps of 280 transit officers patrol stations and trainyards, McLean said. A total of 22 people have been arrested this year for destroying property.

An antivandalism campaign presented in Washington-area schools has helped reduce Metro's maintenance and security costs on the trains and buses to less than $100,000 a year, according to Williamson. Seven years ago, the costs amounted to $500,000 a year, he said.

Each night Metro cleaning crews, like soldiers on a search and destroy mission, walk through the trainyards in Brentwood and New Carrollton where the trains are stored. Using a strong chemical solvent with the predictable name of "Graffiti," they dissolve pungent puns and love pledges. Seats beyond repair are replaced. Walls are scrubbed. Where the solvent fails to clean, a flick of a crew member's pen twists a crude suggestion into a silly riddle.

During conventions and the football season, the graffiti increased, workers say.

"When the Redskins lost to Dallas, I can't tell you what I saw," laughed Kenneth Cook, a cleaner at the New Carrollton trainyard with Nieves. "There was 'Deadskins' and all kinds of stuff."

Another event that produced a surprising spate of graffiti was the gospel convention at the D.C. Aromory, they said. The call letters of a local religious radio station, WYCB, adorned so many seats that a Metro official sent a set of cushions to the station.

During political rallies, some graffiti writers have used the walls of outdoor Metro stations to promote their cause, according to Ralph Smith, director of Metro's custodial services. Sandblast crews were rapidly dispatched to erase the protest.

"We had two major occurrances," Smith said. "The Communist Workers Party Five. that was [spray painted] in three-foot-high letters all over the place in about five stations. It takes three hours to clean something that big." The other most prevalent graffiti protest was "Free the "Armenians," he said.

As ridership increased, the trains have become rolling trash baskets, the cleaners said. Food containers, dirty Pampers, newspapers and marijuana cigarette butts litter the cars daily.

Walter James, a mechanic at the New Carrollton trainyard, has noticed that one particular artist has made the train cars his personal, rolling canvas for his landscapes and portraits.

"He's pretty good, too," James said., nonetheless, adding that Metro still quickly removes his art work.

Metro officials said no figures are available on how many seats have been destroyed or damaged by vandals and graffiti writers. Nieves, however, believes that graffiti writing is increasing.

"I think what attracts the kids are the clean seats," she said.

In the past eight months, James said he has replaced so many seats "it's gotten to the point where we can't keep up."

Previously, all graffiti-scarred seats were replaced, he said. New seats were put in while the old ones were repaired. "Now we just remove the offensive seats," he said.