There aren’t many examples of contemporary bluegrass bands crossing over into more widespread popularity. In fact, there might be only two: Allison Krauss and Union Station, and the Yonder Mountain String Band. But while AKUS tends to attract an older and more traditional crowd, Yonder Mountain has built a loyal following on college campuses, regularly plays mainstream venues, and was even on the bill at the 2008 Democratic National Convention, which took place in…