No, thanks

That's what SEPTA says to questions about whether it is considering a move like those taken by the New York and New Jersey transit associations (see, e.g., this or this) to institute random bag searches.

"For us, I don't think random police activity is highly favorable [to security]," said Jordan. "I'd rather have our officers out walking a beat than concentrating on doing package searches in a couple of major stations.

The argument made in the article is that Philadelphia is not an obvious terrorist target in the way that New York or DC might be; stronger to me is the argument that such searches inconvenience commuters while likely doing nothing to stop a determined terrorist (turned away? try the next stop down). Experts also point out that public transit, by its nature open at multitudinous points, is very difficult to protect at, say, the level of airport security.

A second article discusses the use of bomb-sniffing dogs, both in general and in transit systems specifically. Sounds like almost exactly what the permeable-system problem needs, but it's hard (and expensive) to train enough dogs for the potential scale of the job.