Amtrak announced today that it will cut a fifth of its train service and lay off 5,500 workers in an effort to stem mounting operating losses. Amtrak cited a possible cut in its federal subsidy and intense competition with low airline and bus fares as reasons for cutbacks. Starting February 1, Amtrak will reduce the frequency of trains on several routes including its busy Northeast corridor service between Boston and Washington, D.C. Slated to be cut completely after April are the Chicago-Milwaukee, Chicago-Grand Rapids and San Jose-Sacramento-Roseville routes. Overall, 530 stations won't see the Amtrak's silver trains, officials said. Amtrak has always depended heavily on federal subsidies -- $952 million this year -- but has faced mounting pressure to reduce them. Amtrak lost $76.2 million in fiscal 1994, the fifth straight year it has been in the red. Ridership fell about 300,000 from 1993. More cuts will be announced next year.