BY LAW, BY ADMINISTRATIVE regulation and now by a court opinion, the difficult and costly challenge of providing public transportation for millions of handicapped people is formally a federal government responsibility. The federal judge has upheld sweeping regulations requiring changes in mass transit systems to accommodate the disabled. Though this decision by U.S. District Court Judge Louis F. Oberdorfer is considered a victory by the coalitions of handicapped people who have fought for regulations, it addressed a relatively narrow question.

What the court upheld was the authority of the secretary of transportation to pass regulations enforcing a law. Much tougher -- and up to Congress to figure out -- is how much all citizens are willing to undertake and pay for. The law is broad indeed, requiring that the handicapped not be "excluded from the participation in, be denied the benefits of, or be subjected to discrimination under an program or activity receiving a federal financial assistance." That's an unqualified order that should have been tempered to reflect the limits of what is possible. But it wasn't limited, and it demanded rules for enforcement by the Department of Transporation.

The ensuing regulations require that all new federally financed public transportation facilities be accessible to the handicapped and that the refitting of some existing facilities be undertaken over periods ranging from 3 to 30 years. The biggest impact -- make that read cost -- will be in large cities with old rail systems. Estimates for all of this work run anywhere from $3 billion to $7 billion and do not take into account the physical disruption involved.

On top of this, nobody knows how well various changes would work, how many handicapped people they really would accommodate -- or just how far society is willing or able to go to make the daily living of disable people as much like that of others as possible. To the handicapped, it is a matter of rights and of dignity -- not of "seperate but equal" facilities. But to Congress and all taxpayers, it is a matter of money, technology and timing that will take great amounts of understanding, patience and compromise to address.