SITTING IN A THREE-MILE BACKUP at the Bay Bridge last weekend, on my way home from the last official summer weekend at the beach, I had a vision, an out-of-body experience, a spiritual awakening of the sort that makes millions for aging movie stars. I saw the future, and it is a . . . beachtrain.

Every Friday afternoon, from Memorial Day to Labor Day, sleek, modern silver trains will leave Union Station and head east to the beaches. They will depart every hour, on the hour, deeply air-conditioned, always making clean and timely getaways, from 5 p.m. until midnight.

The passengers will be carrying one or two lightweight canvas bags -- no briefcases -- and will board in an orderly but casual fashion. There will always be more than enough cars on the train; everyone will have a comfortable, adjustable seat that will not have to remain in the upright position and that will not have a tray table. No one will cut in line, no one will have to save a place for a friend, no one will have to ask if a particular seat is taken. Reservations will not be necessary. And although the train will be extremely cold from air conditioning, the two cars in the back will be reserved for those riders who want to open the windows and stick their heads, arms and legs out in the breeze.

Most passengers will have season passes priced modestly, say $100 for June, July and August. Others will purchase tickets once the train is in motion, but the conductors will not come through to collect fares until everyone has a cool drink in hand.

The train will head due east on a newly built track that cuts across the farms and meadows of Maryland in a way that angers no residents of those picturesque small communities. It will glide smoothly toward the Chesapeake Bay where the train will cross water on its own special bridge, slowing so that passengers will be able to admire the lights of automobiles in the seven-mile backup at the Bay Bridge.

In the cars in the front half of the train, gin and tonics and club sandwiches with potato chips will be served gratis. In the cars in the back half of the train, there will be ice-cold beer from the bottle and soft-shell crab sandwiches on white rolls served with french fries. In the last two cars (the ones with the open windows), there will be hot dogs, hamburgers, draft beers and a radio tuned to the nearest available baseball game. Only in these last two cars can passengers play Trivial Pursuit.

As the sky gets darker and the train flies onward, friendships will be made; interesting and well-informed arguments will be begun, in which all parties will be given their say and which will end on a friendly but conclusive basis. No one will mention office politics, terrorism or network evening news, but there will be lots of stories told about summers past, about music from the '60s and about the heat back in the city.

At the end of the 2 1/2-hour ride, the train will pull slowly into a simple old-fashioned station at the Indian River inlet, where spouses and friends will be waiting at outdoor tables. For those without a ride, small, canvas-topped shuttle buses, seating 20 and with destinations such as North Shore, Silver Lake or Bethany will take passengers to shingled cottages or frame houses. There will be no high-rises, no condominiums, no complexes named Many-Tiered- Humonga-by-the-Sea.

All the beds in the houses will be made and will have fresh sheets and no sand in them. There will be cold drinks in iced glasses waiting on the porches and crabs steaming in a speckled pot in the kitchen. Oldies will be playing softly on the stereo, the speakers dragged out onto the decks.

None of the married couples will be having a fight; none of the single people will be having a crisis; no one will want to share any "true feelings" with anyone else.

Everyone will stay up late but plan to get up early. No one will speak about the trip home.