PRESIDENT CARTER has before him one of the best legislative achievements of Congress on behalf of the District of Columbia this year: a bill to stop the richest taxpayer-supported gravy train in town. We refer, as you may have guessed, to the police and fire fighter disability pension system. The pension measure would stabilize the currently shaky financing of pensions for municipal judges and school teachers and administrators as well as police officers and fire fighters. For these important changes to take effect, however, the bill must be signed by Mr. Carter before it dies of a pocket veto.

The bill is not, as some White House advisers reportedly thought at first glance, a fat federal spending bill.On the contrary, Congress has acted responsibly to put a halt to an underhanded giveaway program of its own making. The idea is to tighten up controls on pension abuses that Sen. Thomas F. Eagleton (D-Mo) once labeled "by far and away the premier rip-off system in the United States . . ." In addition to this crackdown, the congressional agreement would, it is true, authorize the spending of up to $65 million a year in federal funds for 25 years to cover retirement payments that up to now have been perilously tied to the city's operating revenues. But without a sound financial arrangement and curbs on pension abuses, the price tag for the system as it now operates is likely to be much greater than the amount authorized in the bill.

What Congress has approved in this instance is similar to what the president has suggested for another part of the District's budget. When he signed the last D.C. budget bill, Mr. Carter called for the development of a sensible formula for the annual federal payment that compensates the District for tax-free U.S. land holdings here. Any federal-payment formula should include a sound pension-fund system. In fact, the White House task force on District affairs should work with the next mayor to coordinate the two payments. But that work cannot begin - and the hard work put in by Congress on the pension bill will come to nothing - unless Mr. Carter signs this measure.