I got a bullet train to Kumamoto. It was phenomenally expensive but pretty awesome, accelerating stunningly quickly to a pace that British trains do not customarily achieve, and while a bus would have been about a third of the price, it would have taken about three times as long.
The train from Kumamoto to Aso only goes as far as Higo-Ozo at the moment as the line was damaged in an earthquake in 2016. And Aso itself is not nearly as accessible as usual, with a lot of trails closed. I'd hoped to go up to the crater rim, but barriers across all the roads and paths heading that way were pretty unambiguous. So I hiked up to Kijima-dake and at least got some pretty good views of the ash-covered surroundings of Naka-dake, the active vent.