I won't agree or disagree with the speed comment, you could very well be correct.

As for powering by solar in Japan (and any other currently electrified system), I would guess that's easily done right now by changing how their power is generated; and that doesn't require a change in the system, just the generation. In japan around 66% of their rail is already electrified (https://en.wikipedia.org/wiki/Railway_electrification_in_Japan look at the summary box showing total miles and electrified miles). So I'm still skeptical that a conveyor system is the answer vs adding more electrified rail in that same strip of land and powering it with solar generation. But again, maybe there's something to be gained with such a different engineering solution per my OP.

And while you're spot on for the US (less than 1% from my google search) a conveyor won't solve it sadly unless there's something about that which makes it cheaper to deploy then adding a catenary system to our current railways.