NO MATTER HOW impressive Metro's record had been, news of the first reported sexual crime on Washington's subway in its three-year history is depressing. True, the subway has been remarkably safe; but the harrowing experience related by an 18-year-old Lanham woman this week cannot be dismissed as something that was bound to happen in time. Not only was the women's ordeal terrible, but this assault by a man in an otherwise empty car on a train coming in from New Carrollton continued unchecked through several station stops.

Metro transit police point out that even though the stations and trains have been unusually free of serious crime, it is the public perception of safety on the subway that is important to its success. That is why the system has a 250-member police force of its own and millions of dollars worth of closed-circuit cameras on station platforms and mezzanines, though not in the subway cars themselves.

By and large, the police officers are in evidence and their presence surely must be a deterrent to crime. Still, there was no officer on that train Monday evening, even though the trains that left before and after that one were patrolled. As the subway's routes and hours of operation increase, the police force, too, needs to grow. Shouldn't there be an officer on every train?

Another step, proposed last year by the International Brotherhood of Police Officers, would be to let off-duty police officers ride the buses and subways free, since an officer is technically on duty 24 hours a day and thus could intervene in emergencies. Present policy permits free boarding only for uniformed officers and on-duty plainclothes officers with temporary passes. Exchanges of free rides for protection have proved popular in other cities, and it's worth a go here.

Obviously, there is no reason for public panic or abandonment of the system and no likelihood of it. The subway continues to be a relatively clean, fast and pleasant way to move through the region -- a special source of local pride.Efforts to make the system the safest in the country should not diminish as the task becomes more complicated and costly.