In this file photo taken on October 2, 2012 Madame Tussauds waxwork models of Swedish pop group Abba are displayed at a roller disco in the Renaissance Room in London. (Photo by Adrian DENNIS / AFP)

 They famously vowed never to reform, to the disappointment of their legions of fans. But nearly four decades after disbanding, Swedish superstars ABBA were on Thursday expected to announce a