White Barrow is mentioned in the 3rd mission of Season 1; And I Alone, Escaped. It is said to be a place that existed before New Alice Springs and the tunnels underneath New Alice Springs predate both New Alice Springs and White Barrow.
One of the game’s most closely held secrets is that the towns were all created to serve a particular purpose for White Barrow and the Dustcaps are the descendants of the support staff that used to keep them connected. In game terms this ties in to the eventual trading network beneath the towns with each location having a different speciality.