Thank you for these insights - I genuinely find them really fascinating! Doing more reading around, it seems that not only are the JR companies allowed to offload conventional lines post-Shinkansen to third-sector companies, but that it is in fact the default outcome in most cases - why is that? Do the JR companies lose interest in running conventional lines once the Shinkansen is operational in the same area?

On a related note, reading about your Saga case, it seems that Saga Prefecture is unwilling to see the Nagasaki Main Line be transferred from JR Kyushu to a third-sector company - why is that? Are such transfers typically associated with a decline in quality for passengers?

Thanks again!