With New York introducing baggage checks on the MTA, the CTA tattler asked the obvious question: "Are we next?" The post sparked spirited debate among commenters, particularly about privacy in the face of screening. Slate explores that very topic in its latest "Explainer," noting a number of variables affect the legality of such an operation. Meanwhile, revelations that the man shot and killed by police yesterday in London's Underground had no terrorist connections can only underscore how difficult the problem of transit security really is.