Here's some required reading for the first day of school: "A Tale of Two Transit Systems." In the D.C. suburbs, it's the best of times, thanks to last week's groundbreaking of the $5.6 billion Purple Line, the 16.2-mile light rail linking Bethesda to New Carrollton. In Baltimore, it may not be the worst, but the contrast is still painfully obvious. Not only is Charm City bereft of its own light rail project, the east-west Red Line from Woodlawn to Johns Hopkins Bayview, but the alternative provided, the less-than-one-fortieth as expensive BaltimoreLink, is likely to cause significant upset today. Why? Because the Maryland Transit Administration's reorganization of city bus lines is expected to catch hundreds of students by surprise on the first day of school.