IF YOU'RE THINKING of seeing the country by train this summer but have not made reservations yet, you'd better hurry-and keep your schedule flexible. Thanks to the gas squeeze, many of Amtrak's long-distance trips are heavily booked for weeks to come. Last month, according to Amtrak, 756,246 requests for reservations had to be turned away.

This may be frustrating for summer travelers. It delights everyone who wants to save the costly, sparsely-used routes that the administration has planned to cancel next fall. Under growing public and congressional pressure, Transportation Secretary Brock Adams last week agreed to continue any of those trains that gain enough riders and revenue. People will "vote by buying tickets," he said.

Using ridership as a referendum is fine in one respect. Any train that is not popular this summer should be killed without another thought. However, a surge of business in one atypical season will not say much about the long-run prospects for long-haul passenger trains. The record of public abandonment of the rails is too immense to be that easily set aside. Last year, for instance, the Grand Canyon counted nearly 3 million visits. How many tourists used the train to Flagstaff? Only 15,000 to 20,000. And how many people used the stations next to Glacier National Park? Perhaps 6,000 - while there were 1.5 million visits to the park.

A vast increase in vacation traffic this year would not really prove that public preferences have changed. In fact, if this summer's train travelers were really polled, we suspect that many, besides mentioning as problems, would say they have come aboard because they want to ride the Southwest Limited or the Crescent once (or possibly once more) before it vanishes. That gets to the heart of the problem. Trains may be a sentimental favorite and a once-in-a-lifetime joy. But for most long-distance travel, they are no longer the most convenient or economical way to go. Buses are cheaper. Planes are much faster - and now less expensive, too.

Consider just a few more figures. Amtrak's operating subsidies now exceed $500 million per year. Some of that helps sustain service that has proved its popularity and worth-along the eastern seaboard, on the West Coast, around Chicago. But under the House Commerce Committee's formula, which Mr.Adams is now willing to accept, much less well-used trains could be saved if they lose no more than 7 cents per passenger mile. That may sound small, but it adds up to a public subsidy of $70 per passenger ( or over $10,000 per train) for a 1,000-mile trip. It is a high price for preserving some sentimental favorites that have become almost everyone's second or third actual choice.