THOUGH METRO officials have been pleasantly surprised by every ridership response to additional subway service so far, they're not at all sure what to expect as Sunday runs start today. It is all well and good to cite the importance of mass transit to the world's energy supply, but the fact is that running those trains from 10 a.m. to 6 p.m. is expensive; and the locally elected officials who make up Metro's board of directors -- already under constituent presure about taxes -- are not eager to commit money to a frill program.

So the addition of Sunday and holiday service has been approved on a "use it or lose it" basis. It isn't a matter of breaking even through fares -- for that isn't the case at any time of the week, nor is it the test for running mass transit service. Still, there will be little point running empty cars around the region every Sunday.

For starters, of course, many Redskins fans will be taking advantage of the service today, but shuttling sports fans to the stadium is not enough justification for the extra expense. Hopes are that the subway will be used by church-goers, tourists and residents coming in and out of downtown to shop or visit museums. Surely the motorist who has circled the Mall with a carload of kids, searching in vain for a place to park, should welcome the chance for an air-conditioned subway ride to the Smithsonian.

Someday, the trains may run 20 hours daily -- but for now, in expanding a most popular service to 356-days-a-year operation, Metro's subway system has done all right for a three-year-old.