[The Oslo to Copenhagen] corridor is the most densely populated stretch of Scandinavia, home to 8 million of the region's 20 million people. An ambitious plan aims to link up its cities, medium-sized by global standards, across three different countries to form a single megalopolis - '8 Million City'.More at Big Think, which notes that expansion with a side link to Stockholm would create a 12-million-person megalopolis. Personally, I would argue with the hypothesis that "next-level development is hampered by the demographic dispersal in small, relatively isolated urban centres," but obviously there's big money at stake here.
The glue that will hold this megacity together? A high-speed train link that should reduce rail travel time between Oslo and Copenhagen from 7.5 hours today to 2.5 hours by 2025.