In an 11th-hour attempt to prevent a massive shutdown of Pacific North-west railroad service by the Milwaukee Road, members of the House Commerce Committee voted yesterday to provide federal aid to keep trains running for 45 more days.

In a unanimous voice vote on Wednesday night, the Senate had approved similar aid for 60 days, and congressional sources said yesterday that they expect full House action on the proposal after the Memorial Day recess next week.

The Senate version probably will be brought up on the House floor to expedite action because a federal judge in Chicago is scheduled to rule next week on whether the 9,800-mile Milwaukee line should be trimmed virtually overnight to a 2,500-mile system concentrated in the Midwest.

Under a plan by Stanley Hillman, trustee for the bankrupt Milwaukee Road (its formal name is the Chicago, Milwaukee, St. Paul p Pacific Railroad), a shutdown of most lines west of Minneapolis is necessary to halt staggering losses by the Milwaukee, the nation's seventh-longest railroad.

Losses are building at a rate of $500,000 a day. Hillman told a Senate subcommittee this week. The Milwaukee lost $100 million between 1975 and 1977 and another $82 million last year.

"To attempt to operate the entire Milwaukee Road for the balance of the year will require $80 million to $90 million more than the railroad itself will be able to generate," he added.

A much smaller system could require "only" $20 million, but 2,800 of about 10,000 rail jobs would be lost and service to many communities and shippers along Milwaukee routes would be stopped.

In recent days, however, lobbying by Milwaukee employes, shippers, state officials and organized labor has convinced some Department of Transportation officials that there are serious questions about Hillman's shutdown plan - which could take effect next month.

Sources said DOT Secretary Brock Adams would recommend that President Carter sign legislation to extend federal aid to the Milwaukee so trains could keep running while studies are completed on what parts of the Milwaukee line should be kept intact.

The 23-15 vote in the House commitee yesterday was along party lines. Although U.S. District Court Judge Thomas McMillen has announced plans to rule next Tuesday and whether the shutdown should begin, congressional sources said yesterday that the judge might delay an announcement in view of a "clear indication" that Congress wants to keep the railroad intact at this time.

The Senate bill was sponsored by Sen. Max Baucus (D-Mont.), who noted that if an embargo on Milwaukee Road shipping is approved next week, the Interstate Commerce Commission would have to step in and order other lines (mainly the Burlington Northern and Union Pacific) to continue necessary services.

State officials and Milwaukee employes have argued that such a step would eliminate any chance that the Milwaukee Road could be started up again later, and they contend that Hillman's plan errs in attributing most of the line's losses to service west of Minneapolis.

The congressional bills direct the DOT to provide aid under existing rail-aid authority to cover losses during the 60-day period. But Baucus noted that shutting down much of the railroad also would be costly. Unemployment aid could cost more than $30 million in the first year for affected workers, and the ICC estimated a cost of $25 million to direct other lines to provide service to Milwaukee line customers.

Baucus wants to provide time for shippers and employes to complete proposals to buy the Milwaukee lines west of Minneapolis, from which they would be precluded if a shutdown takes place. The DOT is working on studies about the Milwaukee Road's future, and a report is due in about a month.

House legislation to provide new aid to the Milwaukee was sponsored by Commerce Committee Chairman Harley Staggers (D-W.Va.) and Transportation Subcommittee Chairman James Florio (D-N.J.).