THE ANNOUNCEMENT by Amtrak that it is canceling more than two dozen passenger trains and increasing fares on most of its routes is hardly a surprise. Some major change of this kind was inevitable once Congress appropriated about $47 million less for the corporation than it estimated it would need in fiscal 1978 to maintain the existing level of service.The changes announced Wednesday are intended to make up about $28 million of that difference and to hold Amtrak's deficit next year to about a half billion dollars.

Most of the trains being eliminated run in the Boston-Washington corridor, the only part of the Amtrak system that shows any sign of ever being economically self-sufficient. We can only assume that Amtrak's management has made a careful calculation that the large number of trains it is cutting from the schedules - mostly on weekends or on shorter runs - will not substantially reduce the number of passengers carried along this corridor. The Metroliners have been the one great success of Amtrak, and they ought not be jeopardized to keep trains running in other parts of the country; particularly on those routes that remain in the passenger system largely because they are the pets of powerful members of Congress.

The bulk of Amtrak's deficit, of course, is piled up by the long-distance trains, where the speed of flying and the ease of driving have made other means of travel more attractive. But the success of Amtrak's cut rate fares to Florida from the Northeast this summer suggests there is still a market for some long-distance passenger service. Unfortunately, neither Amtrak's equipment nor its personnel seemed up to the increased number of passengers those fares attracted. There is an opportunity now - given the cost of air travel and the heightened public awareness of the energy shortage - for Amtrak to woo some vacationers back to the trains. But it is not going to be able to do so with inadequate service and uncomfortable accommodations. And if it can't attract more passengers, the choice in the future is going to be between more reductions in service of the kind announced last week and higher congressional subsidies.