WHEN AMTRAK WAS created by Congress in 1970, the theory was that a well-run system of intercity passenger trains, bolstered at first by federal funds, could eventually be operated at a profit. Spokesmen for the railroads, most of which had given up on passenger trains some years before, countered that that theory was unrealistic, especially as it concerned long-haul routes. It turns out that the railroad people were right. The report to Congress Monday by Secretary of Transportation Brock Adams on Amtrak's past record and future expectations confirms as much. Amtrak has turned into a financial nightmare.

True, Amtrak has been successful in attracting passengers back to the trains. It carried about 40 percent more riders last year than it did in 1972, for instance, and collected twice as much in fares. But that was achieved at a considerable price. Amtrak's operating costs were up 172 percent and it now spends more than $2.60 for every dollar it takes in at the ticket window.

It may also be true, as many friends of Amtrak insist, that the system never had a fair test. It inherited, and has been steadily replacing, unsatisfactory equipment. Its trains have often been handicapped by the refusal of some railroads to give them priority on the tracks over freight trains. It has been forced to slow down its trains and sometimes operate far behind schedule because of inadequately maintained road beds. And it has been required to operate some trains for reasons of a political character that make no economic sense.

Despite the possibility that Amtrak's performance could improve if all those troubles were overcome, the time is here for Congress to face up to the future of railroad passenger operations. Secretary Adams predicts that unless some basic changes are made, Amtrak will need about $1 billion dollars in federal subsidy by 1984. We see no reason to quarrel with his projection; it is in line with Amtrak's record. Indeed, the projection; it is in line with Amtrak's record. Indeed, the projection may even be low if increased competition among the airlines results in permanent cut-rate fares of the kind that are now beginning to appear. We find it hard to square that large a federal subsidy with the benefits provided by Amtrak's operations.

Secretary Adams has proposed that Amtrak drop about a third of its present route mileage and sharply reduce the number of cities it serves. Since even this reduced system would run a deficit of around $750 million by 1984, it is surprising that he did not recommend additional cuts.Our guess is that sooner or later Amtrak will be pared back to three small regional systems - one on each coast and one centered on Chicago - with no more than one or two interconnections.

Any reduction in Amtrak is, as Secretary Adams said, "a political hot potato." There will be plenty of pressure on members of Congress to maintain service everywhere it now exists. That pressure has to be resisted and Amtrak's operations have to be reduced until the value of the benefits of transporting people by train instead of bus or automobile - greater fuel economy and safety, less air pollution and surface congestion - come closer to a fair price.