WHEN the happiest man in Washington was a boy, a kid of 6 or 7, he would go down to the Photomac Yards in Alexandria with his father and there he would watch the trains. He would watch them moving and he would watch them couple with a bang and he would watch the assembled train pull out of the yards, headed for what the songs of the era called for-away places. A boy's eye might follow that train a couple of hundred yards down the line. A boy's imagination could follow it forever.

Now it is years later and the boy is Charles Rumsey, aged 40. He is big and he is beefy and he is sitting at the dining room table in his Arlington apartment, studying. The books are on the table because there will be an exam in the morning and Rumsey is studying for a new career. He has been a draftman for Pepco and a bus driver - first for Greyhound and then Trailways and finally Metro and now he is about to become something else. All over the apartment are reminder that Rumsey's heart still belongs to the railroads - books, magazines and a model or two. Rumsey remains an unabashed buff. When he was making the New York run for Trailways, he knew the name of every railroad line he cross, and there wasn't a whistle that didn't tug at his heart.

He tried to work for the railroad. He tried But the era was over. None of the railroads was hiring and when Rumsey went looking for a job there was none to be had. An uncle had worked in the yards and another uncle had worked as a stationmaster, but Rumsey himself could find nothing. Just once, he dound an opening, but the doctors found he had one too many vertebrae and certified him unfit. The man who loved trains drove buses instead and while there was a thrill to that, too, it was not rail. That was the thing - rail.

Now the books are before him and now there is studying to be done for the examination in the morning. After that, there is six more weeks of schooling and six more exams and then, if everything goes all right. Charlie Rumsey will get a certiciate and a pair of keys. The certificate is for framing, but the keys can open and operate a Metro subway car - a train. Charlie Rumsey can look at those books all night. Studying about trains is not a chore for him. Charlie Rumsey is about to become a subway train operator. Charlie Rumsey, make no mistake about it, is deliriously happy.

Already, he has moved a train. He operated one and the approximate spot was outside the Rhode Island Avenue station. The trains are really run by computer and there was an instructor aboard - an old hand from the New York system - and all this was backup aplenty. But if you asked who was running the train at that particular moment, the answer would have been Charlie Rumsey. He was "scared" and "exhilarated" and he was feeling a bit out of place. In fact, after years on the bus, he was looking around for the mirrors. "You get used to the mirrors," Charlie said. "You get used to seeing what's behind you."

Since then he has been in the cab several times and the thrill has yet to wear off. Sometimes, he sits facing all those dials and gauges and cophisticated machinery and they somehow blur and the cabin changes a bit and something wonderful happens. "Sometimes you think you're going to New York," he said. "You're not supposed to think that way, but you just sit there and imagine you're in Grand Central Station and you're about to make the run south." Rumsey stopped there, catching himself. Grown men are not supposed to think that way.

They do. Maybe there is a motorcycle cop who doesn't occasionally think he's on a horse and maybe there's a lawyer who doesn't sometimes think he's Darrow and maybe there's some workaday, button-downed reporter who doesn't think that someday he'll blow the roof off this town - maybe these people exist, but they're dead before their time, and they're not dealing, anyway, with railroads. Railroads, are something special, something that seizes the imagination. I grew up next to the tracks and I can tell you firsthand that a steam locomotive is a filthy contraption that shakes the house to the timbers and deposits soot on the kitchen curtains. I know that, but I lkie railroads nonetheless.

For a time, I thought maybe this was just my craziness. But if you go down to the Smithsonian and stand before the steam locomotive called "Old 1401" you will see something amazing. You will see, in the first place, kids who were born decades after the railroad era stand before the locomotive transfixed, their mouths open, their imaginations working overtime. And then if you look at the grownups you see the same thing. The recorded crossing bell clangs and the whistle "talks" and the sound of a locomotive pulling out the station fills the hall. You can stand there and watch the kids and watch the people and you can see in their eyes that Charlie Rumsey is not the only one who rides to New York and back within the confines of his own head.

There are people who would not talk the way Rumsey does, who would pretend that a job is a job is a job and that all that matters is the money in the envelope at the end of the week. Not Rumsey. He knows that he's blessed.

"This is my hobby," he said fo railroading. "This is my first love. I'm one of the few people who's doing exactly what he wants to do in life."

The next day the happiest man in Washington passed the exam with flying colors.