Congressional conferees yesterday gave Amtrak an $8 million supplemental appropriation and ordered the national passenger rail service to cancel service cuts that were to take effect on Sunday.

Amtrak announced immediately that it would remove notices about plans to discontinue 20 daily Boston-Washington corridor trains and to curtail other trains across the country.

Train cancellation notices had been posted last week because of a prospective $57 million deficit in the fiscal year that started Oct. 1.

According to Amtrak president Paul H. Reistrup, the government-sponsored corporation would need $545 million of government subsidies in the fiscal year to continue all trains.

The Carter administration, however, recommended a ceiling of $500 million and Congress actually appropriated $488.5 million. In the wake of that funding decision, Amtrak announced last summer that it would eliminate certain trains and its board of directors asked Congress for an additional $56.5 million.

The House Appropriations Committee did not act on the Amtrak request but members of the Senate committee backed a supplement of $18 million.

In agreeing yesterday to provide $8 million, members of the appropriations conference committee said Amtrak should make no general reductions in service and should "live within this budget" for the remainder of the fiscal year.

However, the committee also called on Amtrak to "speed up" a process by which specific and potential passenger markets.

In effect, government sources said, the committee action will keep some trains in service now while forcing Amtrak to eliminate other trains in the near future.

Amtrak said, in response to the congressional action, that cuts planned for Sunday would not be made. Some trains eliminated Oct. 30 will be reinstated as soon as possible.

In addition to keeping northeast corridor service at the current level of 120 trains a day, yesterday's decision means that Amtrak will not reduce from daily to four-times-a-week services the Shenandoah, from Washington to Cincinnati, and the Hilltopper, from Washington to Catlettsburg, Ky.

But Amtrak warned, in a statement that it must eliminate $25 million in costs plus an unspecified additional amount to cover inflation.

The most likely early candidate for cancellation is the daily Floridian, between Chicago and Miami, which has been a consistent money-loser for Amtrak. Hearings on curtailment of the train have been held and directors of Amtrak will Nov. 16 meet to study a report based on the hearings.

An Amtrak spokeswoman said last night that other candidates for curtailment of service included the National Limited, between New York, Washington and Kansas City; Chicago-Pittsburgh-Washington: and a train between Washington and Martinsburg, W. Va.

Other routes under study are Chicago-Laredo; Seattle-Portland; New York-Philadelphia; Chicago-Houston; New York-Boston-Chicago; Chicago-Oakland and Oakland-Bakersfield.

The conference committee directed the Department of Transportation to prepare recommendations for Amtrak's future route structure no later than next March 1.