NEXT WEEK, local officials will be deciding whether the Metro board should begin Sunday subway service come fall. If the system's Saturday experiment, which began last fall, is any indication of how people take to weekend subway service, Sundays and subways should go together marvelously. Metro says its subways are now averaging about 72,000 riders on Saturdays, which is more than officials had predicted. Its weeknight service from 8 p.m. to midnight also is attracting a good response-an average of 14,000 riders, or about 5,000 more a night than the staff anticipated. So what are the Sunday prospects?

Metro board members (who are representatives of participating local governments) say unofficially that they think there are enough votes to approve the change, but they prefer this sort of decision to be unanimous. It would cost an estimated $800,000 more in subsidy in the coming fiscal year to provide eight hours of Sunday service, probably from 10 a.m. to 6 p.m. That, when you consider a total projected operating deficit in the vicinity of $120 million, is not unreasonable for the benefits it would bring.

The extra hours would certainly help if more and more gasoline stations shut down on Sundays. There are churchgoers, shoppers, train and plane travelers, museum visitors and people attending events at the D.C. Armory and RFK Stadium who would be only too happy to use the subway system. Metro's probably-conservative estimate for Sundays is 30,000 riders. And who knows, good all-weekend service might well spark new activities and store operations that could improve the farebox returns as well. For all of these reasons, area officials should run the experiment and find out what their constituents think of it.