In the early days of Metro construction, when visions of potential dangers in subterranean stations and corridors loomed large in the minds of future subway passengers, the Metro Transit Authority created its own police force. And when the first trains began running in 1976 the men and women in MTA green and grey fanned out to test their abilities in new territory.

Their goal was to create a high level of police visibility, "to flood the area," according to Inspector Robert Keahon, "so that everywhere passengers and any criminal element would turn they would see a policeman."

Since the opening of the subway, Keahon said, that goal has been achieved. In the last two years some 340 crimes - 237 this year as Metro has expanded into the suburbs - have been recorded by MTA police.

Officers Diana M. Dennis and A. Wendell Johnson are two of the more than 200 patrolmen, scouts and undercover police who roam the bus routes and subway stations and trains under the MTA badge. Dennis, who graduated from the MTA training school in August, is a novice to police work, while Johnson, with more than 10 years experience as a city policeman behind him, is an old pro. But just as their backgrounds reflect the diversity of MTA professionals, their brief but intense encounters with certain elements of the riding public are often similar.

"I was headed down to Martin Luther King Jr. Avenue and Sumner Street - the bus driver on the line had sent out a 'silent alarm' - and I didn't know what I was getting into," said Dennis in an interview last week.

"For the first time I realized that I was a white woman, and a police officer, getting on a bus in all-black neighborhood. I knew that by the time I got there, people could have been shooting. And I didn't know what was going down on the bus. As I boarded it, all I could think about was, 'What do you say? What can they ready in your face, what emotion?"

For Dennis, what had "gond down" was a rock throwing incident. A group of juveniles had broken several windows on a Metro bus and had fled by the time she arrived. Instead of a possible shootout, the 28-year-old officer was faced only with making the bus "secure tending to the passengers, making sure they were all right."

A year ago, Dennis worked in an insurnace office, where she transferred paper work onto computers, going home each night only to return the next morning to more piles of new papers. Now, she says, she never knows from one minute to the next what will happen, where she will be.

"I am alone 99 per cent of the time, and I've still got plenty to learn," Dennis said. "I've got to develop street sense, street lingo. So I pay attention to officers and ask a lot of questions."

Dennis is one of several MTA police officers who patrols the streets in special Metro police cars where the buses ramble on their rounds. Others, like Johnson move from Metro station to station, riding the trains, moving among the crowds. Selected patrols overlap the regular force, "where we need them," said Keahon. "We use them for crowd control, revenue control. Wherever we find we have more problems."

The police are specially trained, capable of making arrests in two states and one federal enclave, and they carry guns. As Officer Johnson said last week, "We aren't security guards or special police, we're police officers, and we don't tolerate the violation of the law."

The law, in this case, are the rules of Metro - no smoking, no eating, no spitting, no radios and, most importantly, no fare dodging. With 32 pounds of gear - including revolvers, ammunition, mace cannisters, radios, flashlights, battens and handcuffs - strapped to their belts, Metro police officers work their beats after five months of intensive training at schools in Prince George's County, Virginia and the District.

The major crimes MTA police have to face are fare evasions - there were 42 of them this year compared to 3 last - petty larceny and distruction of property. Other crimes committed on Metro turf are assaults, narcotics violations and vandalism. But, according to Keahon, there has been no change in the type of crime committed nor an increase in the frequency of crimes since Metro has expanded its hours to midnight and opened the Orange line to New Carrollton.

College-educated, Boston-bred Dennis joined Metro with an eye to moving up quickly through the ranks, and her manner contrasts sharply with the "old school of cops" exemplified by Johnson.

Johnson got "his first whack at being a police officer," as he calls it, when he spent four years in the Marine Corps, "chasing prisoners around Rome." After a dozen years split between the Annapolis and Seat Pleasant police forces, Johnson joined Metro 3 1/2 years ago, graduating in MTA's second class. "The idea facinated me about being a policeman in three different states at one time."

Johnson, who has worked undercover on buses throughout the transit system, now weaves a pattern of his own design through the trains and stations from Arlington Cemetery to Foggy Bottom. "No one but me knows where I'll be and when I'll be there," Johnson said.

His attitude toward arrest is simple. "I don't think arrest is always the answer to breaking the law," he said. "With misdemeanors, discretion is important. Being more mellow than a lot of guys, if I can get my point across without arresting anyone, justice is served.

"Sometimes, on an undercover, we are concentrating on a certain problem, like armed robbery. Then we let the smoking on a bus go so we don't blow our cover. You know we see the same things on a bus that you see on a bus. But it gets tricky. You can tell someone to put out a cigarette and people can get real violent. Once I was assaulted by a woman who was eating an ice cream cone on a bus."

Johnson said one of his worst times came a few years ago, when he was still new to the force. "I was at a patrol station on Rhode island Avenue and a bus driver left his route and came up there. His bus was full of school kids from McKinley Tech. They had completely taken over the bus, paid no fares . . . I had my hands full and eventually I had to fight one."

Life is different on the train, however. "We're in uniform and you don't have the problems. We're concentrating on prevention and being as visible as possible. You know, people who ride (the subway) are different, patrons don't tolerate the violation of the law. Trains are extremely clean and if you're smoking, someone's going to tell you to put it out."

Dennis said fare dodges are the major Metro problem. "If you see (a fare dodge) you try to give them the benefit of the doubt. Maybe they don't understand, maybe they're from out of town. But you can tell when someone's going to lie to you. And you give them a chance to pay the fare. If they don't, you put the handcuffs on and frisk them."

On her midnight scout car prowls, Dennis, who is one of 15 women on the force, said she gets the "sleeper on the bus, drunks on a bus" calls. "As a woman, I get some bad responses. But they don't know what I can do, they don't know how strong I am."

Dennis said she has had to get used to the idea that some day she must use her gun. "You know, people see guns on TV" and they don't realize what they really are. I am annoyed by that concept. People don't realize the damage it can do or the potential it has to hurt." Each day she said she polishes and cleans her revolver, as well as the brass and leather gear she wears.

"The only thing I don't like is the hat. It makes me look like a duck. I would prefer to wear the same one the men do.

"You know, we have to act like (the men), they expect that, so we should look like them as well. Because they don't care what you look like, just if you can do the job."