WE'RE WELL AWARE that if all the special task-force reports on Metro system over the years were laid end to end, you'd easily have more than a 100-mile line - but the latest reports we've seen does deserve public attention and discussion. It comes from a group of highly respected experts, and it contains some highly controversial recommendations - including a proposal that the basic regional transit system be designed to meet almost all its costs through regular increases in fares. That's something no transit system in the country is achieving these days; and it's also considered politically unaccceptable by officials of the District of Columbia.Yet the alternative is one whale of an operating deficit in the years to come.

According to the report, issued by the Washington Center for Metropolitan Studies, if Metro and its participating jurisdictions continue their present fare and service policies - and if projected ridership slips by as much as 20 percent below 1990 estimates - the annual operation deficit then would be $500 million. Now, it's all well and good to hope that, somehow, all the taxpayers and their governments will pitch in valiantly to meet this financial challenge. But you have to wonder. Fares must be raised, and they must bear some realistic relationship to operating costs. As it stands, the fares have been going up at about half the rate of inflation. Moreover, every fare increase has been bitterly contested, especially in the District.

Instead of subsidizing everyone's fare as a way of helping those riders who deserve a break - school children, elderly, low-income residents - the task force recommends that each local jurisdiction underwrite its special subsidies. District officials argue that that stigmatizes people and that low fares for all should be a policy principle. But obviously there's no such thing as a free ride; either District riders who can afford higher fares - based, say, on an operating cost index - pay them at the farebox, or the deficit has to be made up by the governments (which means taxpayers anyway).

To help keep costs down, the report suggests that lcalities provide their own subsidized services on routes that Metro deems too costly to include in its regional network. In effect, then, Metro would skim off the best service, the routes with the greatest rush-hour ridership. Whether or not you consider that to be good public policy, it may well be that there aren't great economies in a series of separate regional-local arrangements. The report does acknowledge the possibility that even if fares are raised regularly, Metro still might need some subsidy. It suggests that one possible source would be a regional tax on parking spaces.

But just how would all of these moves affect ridership? The report emphasizes studies showing that the number of rush-hour riders is more likely to increase in response to good service than it is in response to lower fares; and that higher fares produce more revenues in a trade-off with a relatively small decrease in riders. What individual residents consider "good service," however, is likely to be tha closest thing they can find to door-door transportation on a good schedule. Thus, too rigid an adherence to the policies suggested by the task force could be selfness, and suggesting additional ways to keep down expenses, such as the use of part-time drivers, the report has outlined some terrily difficult choices that face - and may possible split - the governments of this region.