CONGRESS HAS just moved effectively to stop the richest taxpayer-supported gravy train in town. We refer to the police and fire disability pension system - the one Sen. Thomas F. Eagleton (D-Mo.) once labeled "by far and away the premier rip-off system in the United States, second to none." Thanks to this underhanded giveaway program, created by Congress in the first place, the biggest pre-retirement decision for many police officers or firefighters has been which ailment or injury to claim for a fat, tax-free two-thirds of active-duty pay - and never mind how much pain the claim might wind up inflicting upon taxpayers. Never mind, for that matter, just how closely the claimed disability might have been connected to the performance of official duties.

No wonder a few eyebrows went up when a House Appropriations Committee report earlier this year showed that 81 percent of all retired District police officers and firefighters in 1976 were drawing disability beneifts. Under the current law, a retiree is determined to be either fully disabled or not disabled at all. With no pension fund to cover these and other retirement payments, officials estimated that by the turn of the century, benefit payments to retirees would exceed the active annual payrolls of the affected agencies. As it stands, the District owes more than $2 billion in future benefits to those already retired or currently on the payroll.

Now this prescription for fiscal insanity would be scrapped under a sensible congressional conference committee agreement reached last week. The conferees, headed by Mr. Eagleton and Rep. Ronald V. Dellums (D-Calif.), have agreed to grant a $1.6 billion subsidy over 25 years, subject to a crackdown on abuses. Among other things, the measure would eliminate a provision that permits full-disability retirement because of aggravated off-duty injuries, and for future hires, there would be determinations based on degree of disability for pension calculations.

In all, the pension system in this bill covers police officers, firefighters, municipal judges and school teachers and administrators. It is fair legislation in that it would not cut back the benefits of those who have served under the old arrangement. With congressional acceptance and President Carter's signature, the pension bill would rank as a major legislative achievement for the District.