IT'S BEEN a long-established fiscal fact in this town that the richest gravy train around is the police and fire disability pension system. Thanks to Congress, which created this underhanded-bonus program, the biggest pre-retirement decision facing a member of one of those departments has been which ailment to claim for the tax-free two-thirds of active-duty pay. The real injury, as everybody knows by now, is to the taxpayers.

So it's heartening to learn that some of the very same members of Congress who either encourage, winked at or ignored the great giveaway program over the years are now working to turn it off. Even Mayor Washington, who usually gets fidgety when you mention taking on the police department (be it pensions, police review boards or department lobbying in Congress) has concluded publicly that something should be done.

A somewhat stronger conclusion was reached some months ago by Sen. Thomas F. Eagleton (D-Mo.), who labeled the pension program "by far and away the premier ripoff system in the United States, second to none." Mr. Eagleton, who is chairman of the Senate subcommittee handling District affairs, is pressing for enactment of legislation that would tighten and improve the system. Similar legislation has passed the House. And now the city board that grants the police and fire retirements is making the same suggestions for improvements. All board members agree, for example, that pensions should be granted in proportion to the severity of disability - not, as they are now, on a disabled-or-not-disabled decision.

The Senate legislation also would severely limit outside income that could be earned by a person on disability; and it would eliminate a provision under which people may retire on full disability because they are found to have aggravated old, non-job-related injuries. These are all pretty obvious moves that should have been made long ago. But in the past, the official attention span has been short, lasting little more than a few days after some newspaper article outlines a particularly glaring abuse of the system.

Now, at least, the congressional support for changes does seem stronger than usual. Rep. William H. Natcher (D-Ky.), who has just asked the retirement board to come up right away with still other proposals for improvements, summarized the mood the other day: "This situation has to be corrected. Maybe I should have done more, maybe the subcommittee or Congress should have done more, or the District government. The point is we've reached the point now where taxpayers are demanding that something be done, and we've got to do it." Amen.