AMTRAK turned 20 this year. When the federal rail system was cobbled together in 1971, passenger train service in the United States looked as though it was destined to join the Conestoga wagon and the paddle-wheeler as a romantic relic from America's transportation past. Over its two decades, however, Amtrak has transformed a disintegrating passenger-rail network into a respectable, albeit still secondary, component of the US transportation system.True, passenger trains were taken off the endangered species list at high cost to American taxpayers. Although the federal operating subsidy has dropped as Amtrak's revenues have climbed, it is still substantial. The rail corporation is perennially near the top of some budget cutters' lists. But Amtrak has proved its importance to the nation's transportation future, especially as congestion on the roads and in the air and heightened environmental concerns are being recognized as downsides to plane and automotive travel. Amtrak deserves to be allowed to pursue - prudently, to be sure - plans on the drawing boards for expanding and modernizing the rail system. Congress rightly included modernization funds for Amtrak in its new transportation bill. Included in that package is $384 million to upgrade Amtrak lines in southern New England. The funds will be used to straighten curvy rail beds, build power substations, and add new high-tech signaling and communications equipment. Money will also be spent to complete the electrification of the tracks between New York and Boston (trains from New York currently must switch to diesel engines at New Haven, Conn.). With the modernization, rail travel time between the cities will be reduced to just over three hours, making it competitive with air-shuttle service when airport commuting and waiting times are added in. If there's one part of the country where passenger train service may soon be able to pay its own way, it's the heavily populated Northeast corridor. The system upgrade will lift travel convenience for millions of people annually, help unjam the traffic around airports, and serve as a rail showcase for other areas looking to the future.