In a sign of Detroit’s continuing challenges, the city’s schools, police stations, fire houses and other public buildings lost power yesterday after an aging underground cable failed, the Wall Street Journal reported today. Within 90 minutes, the Detroit Public Lighting Department with help from DTE Energy Co. began restoring service, and all affected buildings had access to power by yesterday evening, the city said. It also served as a reminder of the city’s problems, even as it nears an exit from chapter 9 protection. “The bankruptcy order doesn’t solve the decades of neglect in our infrastructure, and that is what we saw today,” said Mayor Mike Duggan.