THE SUBWAY CARS still glisten and glimpses of graffiti are few, but Metro's age is showing in glaring ways up and down its lines. No one knows this better than General Manager Richard White, who has warned for years of the effects of aging on the system. Like the farecard machines at the gates, mechanical failures are taking their tolls on service, roiling riders all too regularly. It doesn't take a rail scientist to figure out what's needed most to make the system roll more smoothly; money is the key. But riders cooling their heels on the platforms while peering down the tunnels for their trains should be pleased to learn that some financial relief is on the way.

The federal government is giving Metrorail a $600 million loan guarantee to speed rehabilitation of rail cars and to improve brakes, doors and other equipment that has been causing major breakdowns since spring. Peter Benjamin, Metro's chief financial officer, says the guarantee will enable the system to spend $900 million for rehabilitation of subway cars, escalators and elevators over the next decade. Without this assistance, he notes, Metro would have had to seek help from commercial lenders, adding as much as $20 million to the tab.

The federal program, which provides credit assistance for various transportation projects, included this region among the first five recipients. It allows the federal government to leverage dollars, spending $51 million to guarantee $6.5 billion for the five projects. The help here comes at a good time. With Metro's completion of its original network now that the last two Green Line stations are open, officials can shift their focus to reconstruction of old facilities. That emphasis is essential if the system is to keep commuters and other rail users out of their cars.