When the subway trains of the Bay Area Rapid Transit system rattle through tunnels under San Francisco and over elevated tracks in Oakland, Ray Mok is in control. As BART's principal network engineer, Mok has created one of the most technologically sophisticated public transportation systems on the planet, using the protocols that power the Internet to manage BART's thousands of moving pieces.

Yet Mok's network features plenty of security at key junctures. Critical systems that control the trains sit on a different network that remains physically unconnected from BART's other systems. And he's careful to separate the network that runs BART stations -- including everything from ticket machines to automated gates and escalators -- from the administrative network that powers the PCs of BART employees and that connects to the public Internet. Everything is protected by an extensive web of Internet security software and hardware, including firewalls aimed at fending off hackers and intrusion-detection systems designed to spot cyber break-and-enter artists who make it past the virtual fence.

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