A conductor of a Bay Area Rapid Transit (BART) train looks out his window at the Rockridge station in Oakland, California February 12, 2015. Tens of thousands of commuters on San Francisco's Bay Area Rapid Transit system may have been exposed to measles after an infectious Bay Area resident rode a train to and from work for three days last week, public health officials said on Wednesday. REUTERS/Robert Galbraith (UNITED STATES - Tags: TRANSPORT HEALTH)