>>132755
Mostly just different JRs - JR East stock is generally designed for both 50/60Hz so it could run but JR Central/West/Kyushu trains are 60Hz only (slightly smaller transformers afaik), though Kyushu Shinkansen platforms are only 8 cars too.
But yeah. It'd be a nightmare to timetable and there'd be no operational benefit from linking the two JRs at Tokyo station.