FOR YEARS now, the Red Line from Union Station out into Maryland has terminated at the above-ground platforms in Silver Spring. But as of Saturday, passengers there will start seeing a tunnel at the end of the light: trains will keep on going down the track, burrowing swiftly into the deepest station in the Metro system -- Forest Glen -- and onward to Wheaton. It's the first expansion in four years, and brings the total of rail stations to 63. Officials estimate that this addition of 3.2 miles will add 22,400 passengers to the system's daily average of 513,000. As members of Congress and officials of the participating state and local governments long ago recognized, "America's subway" is serving two important transportation functions: it is not only a popular national tourist attraction and service but also a major daily mover of federal employees and local residents.

All along, the patience of residents and businesses along the proposed system -- those who have been waiting longest for service -- has been extraordinary, given the phasing pattern of construction and delays caused by money limitations and/or political/geographic difficulties. Still to come is the most important link in terms of service to those who rely most on public transportation: the Green Line. Barring any more delays, the rail system could pick up as many as 137,000 more riders when it opens 11 more stations by the end of 1993.

As always, financing for Metrorail is no sure thing -- especially now, with new pressures on the state and local governments and with more demands for money to maintain the system's rail cars to serve the growing number of riders. Metro has had to borrow $25 million this month because the District of Columbia government couldn't make its quarterly payment to the regional system.

Like all public transit systems in the country, Metro doesn't make enough money from fares to cover its operating expenses; it does better than many, though, bringing in about half of its operating budget. And the history of state and local governments' financial commitment to the system has been one of steady payments up to now. Past presidents, Congresses and state and local leaders from both parties have come through with the necessary funds. This commitment cannot be allowed to flag now, as this unique intergovernment venture finally reaches its most patient taxpaying partners.