Faced with a $118 million cut in federal aid, Amtrak yesterday announced a nearly 10 percent cut in service.

Effective Oct. 1, three entire routes will be dropped and several more will be shortened. At the same time, a handful of routes will receive less frequents service, including some of the most popular routes in the Northeast.

Overall, however, the bulk of Amtrak's nationwide rail service will remain intact as Amtrak tries "to run the maximum number of trains and provide the maximum amount of transportation service" despite adverse finances, said Amtrak President Alan S. Boyd.

Nonetheless, Washington-area train travellers are likely to suffer substantially from the planned cuts.

The 13 Metroliner round-trips that Amtrak now makes between Washington and New York will be reduced to 10 round-trips a day.

Although Amtrak does not know exactly which Metroliners will be cut, officials indicated yesterday that the most likely trains to be dropped are those that run in the afternoon, when travel is not heavy.

Additionally, Washington-area residents no longer will be able to travel by train to Harpers Ferry, yw. Va., on weekends because Amtrak plans to eliminate the weekend Blue Ridge run that goes between Washington and Martinsburg, W. Va. The weekday Blue Ridge will be kept, however.

Completely eliminated will be the lightly travelled Shenandoah run, which goes between Washington and Cincinnati via West Virginia.

Two other short routes -- the Pacific International, which goes between Seattle and Vancouver, and the North Star, which operates between Chicago and St. Paul -- will also be discontinued.

One other route, the Cardinal run between Washington and Chicago via Cincinnati, may be eliminated or trimmed to go only between Chicago and Cincinnati. However, the board of directors postponed a decision on that route for several weeks after a legal dispute erupted between Amtrak and the Department of Transportation.

Transportation officials argued that the budget cuts, as approved by Congress, require Amtrak to drop the Cardinal. Amtrak argued it is required to drop only part of the route, the segment between Washington and Cincinnati.

Overall, the cuts are far less than Amtrak had predicted would be necessary to cope with the $118 million cut Congress made in Amtrak's budget for the fiscal year that begins this October.

Amtrak had sought an $853 million subsidy from the government. The Reagan administration tried to reduce the subsidy to $613 million. But after a grass-roots campaign in support of Amtrak, Congress gave Amtrak a $735 million subsidy.

When Congress reduced the requested subsidy earlier this year, Amtrak's Boyd predicted railroad service would have to be cut by 15 percent. But since that time Amtrak has cut its headquarters staff, closed a maintenance facility and changed its food service to trim costs.

Now, instead of receiving freshly prepared meals, railroad passengers will get meals that have been made in advance and reheated, like those served on the airlines.

"Amtrak has had to struggle since it began" 10 years ago, Boyd said in announcing the cuts. "I don't see any great change in the future," he added.

Boyd said that despite a reduction in the number of Metroliner runs, "We are not eliminating or reducing Metroliner service." In fact, he said, the high-speed service may even be improved because, in making the cuts, yamtrak also plans to speed up at least three of the Washington-New York Metroliner trips. It expects to trim the 3 1/2-hour trip to 3 hours by eliminating some of the stops between New York and Philadelphia.