Cutpurse capers

SMART-CARD public-transport ticketing systems let people hop between buses, subways, trams, surface rail and river boats--even when these are operated by different companies--without having to buy new tickets. This undoubted good, though, has ramifications. One is that anyone with access can, by following individual passengers (or, at least, their cards), study precisely where people are going. Companies use this knowledge to optimise services--again, an undoubted good. But many other things, some disturbing to freedom lovers, might also be done with smart-card data.