THE OPENING OF the Orange Line today brings Metro service to Prince George's County and far northeast Washington - and brings the regional rail system closer to maturity. The new 7.5-mile segment, from the Blue Line's Stadium/Armory stop to the New Carrollton station on Route 50 just inside the Beltway, extends the trains into a congested corridor where public transit has generally been poor. Starting this week, getting from Deanwood to downtown (on trains marked Orange/Blue for now) will be a pleasant 20-minute ride. The trip from New Carrollton to National Airport will take 45 minutes - but travelers will not have to fight midtown traffic or worry about tieups on the Wilson Bridge.

For Metro, reaching the Beltway is not just a symbolic advance. The new suburban stations are the first with large parking lots - 1,900 spaces at New Carrollton, and a total of 1,500 at Landover and Cheverly. This will provide the first real test of Metro's ability to persuade commuters to park in the suburbs and ride in to work. Metro's managers will also encourage rail travel by altering bus routes, as on other lines, to feed people to the new stations. Most of the bus changes will occur Dec. 4.

Besides putting more communities on the Metro track, the extension should improve Orange/Blue Line service overall. Instead of wrestling with makeshift switching and repair arrangements, Metro can now use its permanent, on-line service yard at New Carrollton. Mysterious delays and untidy cars should become even rarer - though farecard problems may persist.

For those who live near the new stops, the advent of Metro may not be entirely smooth. Neighbors of the Minnesota Avenue and Deanwood stations are understandably concerned about traffic and parking problems. District and Metro officials may well have to make adjustments here, as in other neighborhoods, as station-use patterns and their impact become clear. The city should also do more to help the small businesses in that area cash in on Metro's proximity. And while some in northeast Washington are wary of Metro, one problem in Prince George's is the outburst of enthusiasm in New Carrollton, where some local leaders are angling to annex the station and the $58-million development project in the "golden triangle" nearby.

Two major effects of Metro's progress are already plain. The system is changing working and living habits in the region - and it is benefiting from its own success. In every jurisdiction, political support for Metro has grown as more people have used the trains and the tracks have come closer to home. There was no audible anti-Metro sentiment in this fall's major campaigns; Fairfax County's congressional rivals actually argued about which of them supported Metro more. In Prince George's, too, the earlier sourness about Metro has gotten hard to find. Despite the county's budget strains, the question is not whether to underwrite the full rail network, but how. That change, too, should be accelerated as the Orange/Blue Line starts working and more taxpayers find out what benefits the great investment in Metro can bring.