As luxury condo buildings crowd the South End’s once-industrial SoWa district, developers have to work harder to preserve the neighborhood’s arts-community roots. So, kudos to this public-private partnership, which covered the eight-acre I-93 underpass in colorful murals by a variety of street artists such as Boston-based Percy Fortini-Wright. The awesome urban park is now brimming with bike paths, outdoor fitness classes, food truck fests and a forthcoming seasonal beer garden from the Castle Island Brewing Co. crew. In other words, Underground goes above and beyond.