SINCE BOTH HOUSES of Congress have now passed Amtrak bills, a lot of senators and representatives can go home and tell the voters how they saved the local train. Some of them may announce this as soon as they step off the plane. And that points to the major defect in the Matrack bills. Both houses voted to keep some trains that few people, including lawmakers, acturally use. Both accepted the idea that each region (but not each state) should have some passenger service for a few years more. That shows that Congress has not entirely gotten over the habit of treating some trains as public pets -- beloved old workhorses that eat a lot and no longnr do much work, but that no one can quite bear to dispatch.

In general, though, Congress has not let sentiment override good sense. Both houses did reject attempts to freeze the entire Amtrack system in its current ungainly state. Some of the most lavishly subsidized routes will be dropped this fall. More important, the fate of other marginal trains is to be determined by their ridership and cost, not their political appeal. This sets up a use-it-or-lose-it test for the nation's travelers. Their response should go a long way toward settling the argument about wheather the current boom in train travel is a temporary reaction to gas lines or a longer-term trend.

Amtrak right now is in transition. It has been a wildly uneven system with some great successes, such as the Metroliners, and some costly mistakes. The administrarion has been trying to cut its losses. Now, while the Senate and House still have to agree on details, both houses have accepted the fact that a leaner rail-passenger network will be a stronger one. They are also providing generous amounts for new equipment and other improvements, plus some strong suggestions for better management. In short, after years of arguing over whether Amtrak has really had a decent chance, it is being given a better opportunity to prove its worth. What senators and representatives should be telling their constituents is that now the fate of those trains is up to them.