METRO, WE OBSERVE, has again been busy at the improvement of its fare structure. Metro's computer-driver purpose is, ultimately, to achieve a fare of perfect equity and symmetry toward all travelers in every imaginable circumstance. It is a noble thought.

In pursuit of it, the new fares effective July 2 will make the rush-hour trip cheaper than non-rush trips on short distances, but more expensive on long ones - except for short trips under the Potomac, which will also be more expensive. Bus connections will cost more for near-in Virginians but less for far-out Virginians, while in Maryland the long ride to Rockville goes up. There's to be a new Metro Flash Pass offering unlimited bus rides within limited zones, but limited subway rides through any zones. Is all that clear?

There seems to be one persistent trend in all of Metro's fiddling with its fares: They aren't getting any simpler. Shortly, we assume, Metro will begin to follow the airlines into the realms of higher mathematics. You can look for the 21-day excursion fare to Silver Spring, but only if you buy your Farecard 30 days in advance; and there will be the special standby fare to Anacostia, with seating on a space-available basis plus further reductions if you bring your own lunch.

But to achieve absolute social equity it will obviously be necessary to integrate the fare structure with the income tax. Simple justice requires a child-care credit, knocking 2 cents off the parents' fare for each child under 6, or 6 cents for each child under 2. There will be the Moynihan - Packwood - Roth tuition fare credit, for people with kids in private schools - a nickel off the regular fare it it's a school with low tuition a dime if it's expensive. There will be a Conable - Fisher, charitable fare cut of a penny a mile for people who give more than $2 a week to UGF and the church of their choice. But that will necessitate a capital-gains surcharge on those travelers whose capital gains last year exceeded either the Metro deficit or the amount of time that they have spent sitting in trains with stuck doors, whichever is higher.

As you stare into this unpromising future, you will doubtless ask why Metro keeps playing around endlessly with its fares. There are two reasons. One of them is the infinite flexibility of the electronic Farecard equipment, which the Metro Board finds irresistible. Engineers call it the New-Toy Principle. Beyond that, there's the serious point that Metro is jointly run by three jurisdictions that disagree on basic fare policy. Virginia concedes that high taxes drive riders away, but it wants to push fares as high as it dares in order to keep any more of Metro's costs from falling onto the local property tax. The District of Columbia thinks that the overriding social goal is to get the maximum possible number of people onto mass transit, letting the deficits fall where they may. Maryland's position lies, uneasily, somewhere between the other two. The combined bus and subway deficit, incidentally, is now estimated to be $100 million in the coming fiscal year.

We also note that the Metro Board has given final approval to its welcome expansion of subway service next Sept. 25. The trains are to begin running Saturdays, and until midnight during the week instead of shutting down at 8 p.m. as they do now. On hours and the general quality of service, Metro is doing well. But as for the fares, we'd rather just throw a flat 60 cents into the fare box and be done with it.