I’m pretty sure all of their new HSR comes from upgrading existing lines rather than building new ones.
Most older Japanese track didn’t meet the grade and turn requirements for Shinkansen track, so much of it had to be built from scratch. That’s why there’s so many HSR stations in Japan named “New <Place>” - they couldn’t get the new track into existing stations.
Current examples would be the Chuo Linear Shinkansen between Tokyo and Nagoya, which is a brand new maglev track that takes a different route from even existing HSR tracks. There’s also the Sapporo leg of the Hokkaido Shinkansen, which I understand to be completely new track.
Most older Japanese track didn’t meet the grade and turn requirements for Shinkansen track, so much of it had to be built from scratch. That’s why there’s so many HSR stations in Japan named “New <Place>” - they couldn’t get the new track into existing stations.
Current examples would be the Chuo Linear Shinkansen between Tokyo and Nagoya, which is a brand new maglev track that takes a different route from even existing HSR tracks. There’s also the Sapporo leg of the Hokkaido Shinkansen, which I understand to be completely new track.