IT IS NOT NEWS--and hasn't been for the last two decades or 20 miles of subway track, whichever came sooner--but Metro has money troubles. The second sentence, of course, is that in every perils-of- Pauline episode, the transit system has been rescued thanks to legislative heroics, silent collections among the worshiping local governments and an apparently long green line of support at the farebox. Still, how much more can the passenger traffic bear in the way of cash up front for every ride? At what point do strapped riders refuse to go the last mile? The news, which is not good, is that the member governments are more and more at odds over how best to ante up.

Only last week, the Arlington County Board formally proposed that Metro depend far more on fares and other revenues and less on subsidies--even if it means raising fares--and never mind that this weekend fares are going up anyway for the third time in 17 months. The county proposal is for fares and other revenues to cover about 75 percent of the bus and rail system's operating costs, with subsidies making up the rest. The current ratio is about 50-50.

The first official "ouch" came from Metro Board Chairman Joseph A. Alexander of Fairfax County, who termed the Arlington proposal unrealistic, which it surely is. Still, in the face of less and less help from the federal government in meeting operating costs, something's got to give, and it will be the rider in at least some more generous way in the near future. That, plus all the other implications of financial constrictions, voter unrest and demands for safe, clean and dependable service, will be the subjects of Metro's annual soul-and-pocket-searching retreat this week--during which the federal, state and local participants convene at the Airlie conference center near Warrenton, Va., to stare at the next five years of transit in this region.

It is not all fares or subsidies, either. At another conference held last week, by the Greater Washington Board of Trade, to examine development on and around subway stations, Metro General Manager Richard S. Page emphasized the revenue potential of commercial, retail and residential projects along rail lines. He pointed out that attractive, well-planned joint ventures by developers, local governments and the transit system can mean solid revenues.

Metro must have such a steady source of operating funds, Mr. Page said, so that it can stop "just passing the hat through eight local governments every year." A regional tax, dedicated to transportation--as other regions around the country discovered long ago-- would help, too. But unrealistic demands on riders or unrealistic demands by one county on a regional system are not the route to financial health for this region's mass transit system.