IN THE AFTERMATH of the London bombings, it was easy to throw around large dollar figures signifying the immense security needs of mass transit systems. Over the next few months, it will be more important to put some of these numbers in perspective and in particular to identify which security investments in mass transit are really worth the money.

Certainly the Washington area cannot claim to have been "unfunded" in this regard. In the fall of 2001, Metro received $49 million for security needs, to which $6.5 million was added from subsequent homeland security budgets. That money has been spent on putting chemical sensors and intruder detector systems in underground stations; on equipping buses as well as stations with digital cameras; and on adding bomb-resistant trash cans and backup communication cables, among other things. Metro now says it needs $143 million more, about half for more of the above -- more cameras, more sensors -- and about half for a backup operations control center. For city officials and federal funders, that backup center should be the clear priority: If the current control center should be damaged, the entire system would effectively grind to a halt.

Another priority, which is not on the official wish list, should be security training. The system's employees should get periodic refresher courses, both in emergency response procedures and in prevention tools, on how to recognize a potential suicide bomber, in particular. Metro should also consider expanding and advertising its emergency response "courses" for passengers, so that regular commuters would know what to do in case of an attack.

There are also some things that Metro should not try to do -- namely, anything that would render the subway system much more difficult to use than it is today. The disruption that would be caused by metal detectors, frisking or a massive police presence is not worth the money, and passengers should not demand it or feel worried that they don't see it. The best security is the most invisible security, as both Metro and its customers should keep reminding themselves.