OBSCURED BY an emotional debate of today, about whether American taxpayers should continue to subsidize intercity passenger trains, is the reality that this coming summer may be the last to see the panorama of America go by outside your window, as you sit back and relax to the sway of the coach.

An era is coming to an end, no matter what happens to the Carter administration's proposal to reduce by 12,000 miles - about 43 percent - the existing and already skeletal, 27,700-mile passenger train system operated by Amtrak.

Unless either house of Congress disapproves the plan within a few weeks from now, Amtrak must restructure its service on Oct. 1. The rail system will be dismembered and rail service to such urban centers as Atlanta, Dalas-Ft. Worth, Cincinnati, Indianapolis, Louisville and Birmingham would be stopped.

Long-distance trains simply are not economical because of labor and material costs involved in hauling several hundred people in a thousand miles or more. If the actual costs of running these trains were translated into fares, instead of subsidized by the general taxpayer, few people could afford the trip. Even on such well-populated trains as those between New York and Florida, Amtrak loses $122 for every passenger it carries. Nationwide, Amtrak fares cover just 37 percent of the costs.

According to Secretary of Transportation Brock Adams, the proposed curtailment would save $1.4 billion over the next fivce years but continue to provide service for 91 percent of current riders.

To train buffs - and they are legion even if they don't ride the rails regularly as paying patrons - the death knell is understandably painful. There is something intangible about riding a train, an exhilarating individual freedom to enjoy motion and scenery and people without doing any of the work yourself.

As Arthur LeMerle of Hyattsville said aboard the final Southern Crescent operated out of Union Station earlier this year by Southern Railway, which had operated passenger trains since 1830: "This train and its people are a way of life . . . it won't be the same." Amtrak took over the Crescent, the last overnight liner that had been operated by a private company, and had increased service between Atlanta and New Orleans from three times weekly to daily.

But Amtrak's Crescent is scheduled for demise within a few months and it is certain that many long-distance trains of today are going to the way of Rockets, Cannonballs and Super Chiefs in earlier times. Other trains will survive, particularly those with heavy patronage in densely populated corriders such as Boston-New York-Washington.

In future years, as the true dimension of the energy crisis becomes known, there may be a drive to restore some trains now scheduled to be cut. Ironically, as government leaders debate Amtrak's future, ridership increased 6.6 percent in the October-January period, the first four months of the fiscal year.

Supporters of Amtrak argue that the system has not been given a chance to prove it can attract Americans out of their cars. With new equipment now coming into use - including the first new sleeper cars ordered for American rail travel in a generation, which also will be the last passenger cars to be built by Pullman - a good case can be made that now is not the time to end the great Amtrak experiment, less than a decade after the first chapter.

Amtrak President Alan Boyd makes the point that trains are not tied to petroleum, since electrified service could be expanded. However, even if every single new old and old railroad passenger car was filled to capacity, there would be little energy impact. Amtrak's equipment could carry, at most, a percent or two of intercity travel in America today. Currently, Amtrak accounts for less than 1 percent.

There is some truth to the suggestion by bus lines that it would be cheaper to give all Amtrak passengers an airline ticket, with less cost to the taxpayers and a savings of energy with better airline capacity rates.

Thus, it is likely that future passenger trains, even if some routes are restored in the next decade or so, will be designed for the very practical role of transporting people on necessary trips rather than carrying them to holidays or vacations.

That brings up the summer of 1979 and a warning to all persons who may be thinking or riding one of the last of the great trains. In brief, Amtrak expects its trains to be packed, particularly during the peak travel months of July and August. This is the time to start palnning the trip and making reservations. If you wait too long, you're bound to be disappointed.

Indeed, Amtrak now is reporting a 40-percent increase in advance bookings over the same period last year. A number of factors are at work, including a strike against United Air Lines and projected gasoline shortages and closed gas stations on Sundays. In addition, some travellers already are planning one last vacation trip before a favourite train expires.

If you haven't taken such a trip or have no favourite train, or if you have a family and they've never taken a family rail trip, you should get to work on one this weekend. For most people it's a thoroughly relaxing time and one that will be remembered.

You'll will have to pay attention to your budget and set aside some extra money, however. Train travel is not cheap, although there are some bargain vacations available, especially when compared with foreign travel. And, if you make reservations late in May for unlimited travel by mid-June on Amtrak's U.S.A. Rail Pass,you'll escape some stiff fare increases scheduled to be announced this weekend.

Even if some of the premiere, sightseeing-oriented trains survive the coming financial crunch at Amtrak and you get another chance to see America from the rails, it will never be as cheap.

Amtrak travel spokeman John F. McLeod says package trips to three of the biggest and most famous national parks - Yellowstone, Grand Canyon and Yosemite - are expected to be especially popular during the coming summer.

In the days before automobiles and mobile homes devoured these parks, they became popular tourist attractions primarily because of luxurious rail passenger service.But all would lose direct rail connections under the administration plan with abandonment of the North Coast Hawatha between Chicago and Seattle, via Livingston and Boseman, Mont.; the Chicago-Los Angeles Southwest Limited, which now stops in Flagstaff, Ariz., and the San Joaquin between Orkland and Bakersfield, Calif., which stops at Merced, near Yosemite.

While bus connections from more distant cities in the future could provide a rail-bus trip to at least Grand Canyon (from Las Vegas) and Yosemite, it won't be the same. Currently,the parks are included in such escorted transcontinental rail tours as that offered by Four Winds, a $2,498-plus, 25-day "Grand Circle Americans."

There also are escorted and independent tours through Amtrak, including three-day stopover packages of Yellowstone, for as little as $74.50 a person, double occupancy; of Grand Canyon, $94; and Yosemite, $129.Included are transfers between Amtrak stations and the parks, two nights of lodging and sightseeing. More expensive packages include meals, and roomettes or sleeping bedrooms are extra.

Another private tour operator, Maupintour, is offering eight holidays that incorporate rail travel. For example, a combined Yellowstone-Glacier parks trip, costing $1,121, covers 13 days and includes 35 meals as well as an evening in Jackson Hole and roomette accomodations on the train. The fare is for departures from Chicago, so the trip to that rail hub is extra.

Amtrak itself offers vacationers more than 100 travel packages in five groupings:

33 independent western tours,providing an opportunity for passengers to work out their own itinerary that could include Disneyland, Grand Canyon, Seattle and Vancouver, and ranging in cost from $7.50 to $644.50.

19 southeastern U.S. tours, also arranged independently, from $24 to $450.

26 "Eastern Escapades," to New England, New York, Philadelphia and Montreal, among other points, from $24.50 to $354.

26 escorted tours, with tour directors and special parties, all in parts of the country, from $219 to $5,868.

29 group tours, customized to satisfy the desires of a church, school or civic organisation, from $11.50 to $439 per person.

For family travel, the best bargain probably is the U.S.A. Rail Pass. Today, a family of four (husband and wife, two children under 12) can make a transcontinental round trip of 6,900 miles via Amtrak for less than 7 cents a mile for the whole family compared with 17 cents a mile, estimated for an intermediate-sized car by the federal government.

Passes can be purchased for 14, 21 or 30 day and are good for unlimited travel throughout those periods. A 14-day pass costs $169, the 21-day pass is $219 and the 30-day pass is $259. The head of household pays full fare but the spouse and children 12-21 pay half fare while children 2-11 pay 25 percent. Travel can begin any time but Friday or Sunday.

But these fares apply only through May 25, after which substantial increases will take effect and remain at higher levels throudhout the summer. Tickets purchased after the date will be $289 for 14-day passes, $379 for 21 days and $459 for 30 days; last summer's rates were $250, $315 and $385, respectively.

If you can move quickly, there is a way to take advantage of the current, cheaper rail-pass fares. Since you are permitted to begin you trip five days after purchase, tickets bought on May 25 would be good for travel starting as late as May 30 and lasting for number of days purchased. Thus, an early June train adventure would be the cheapest for the summer of 1979.

Credit cards may be used to purchase Amtrak tickets and food in diners and club cars. Reservations can be made by calling toll-free numbers listed in telephone directories under Amtrak; the numbers vary according to geography. Many travel agents handle Amtrak trips and in downtown Washington, there is a ticket office at 1721 K St. NW, which probably is best for tours. But tickets also can be purchased or picked up (after making reservations by telephone) at Union Station, Alexandria and the Beltway station at Lanham.

McLeod advises potential travelers to remain flexible in dates for travel. If coaches already are booked on day, ask about the availability on nearby dates. If you want to take bicycles with you for day trips, they must be in a carton (supplied ny the rider or purchased for $4 from Amtrak baggage service).

Finally, with the start of Daylight Savings Time, Amtrak is altering the schedules of many trains. In many cases, the trips willl be scheduled for slightly longer periods than under current timetables, to reflect another sad reality of train travel: They just don't run as fast as they used to because the railroads primarily carry freight and they are fighting now to retain that business, having given up on the passengers years ago. CAPTION: Picture 1, An Amtrak bedroom with upper and lower berths; Amtrak photo; Picture 2, The dome lounge car on Amtrak's San Francisco Zephyr.