I caught the Aso Boy! Kuro to Mt. Aso (mascot: a small cartoon dog). It wasn’t the train I thought I was getting on, but it went to the right place, if not as quickly.
It’s a family sightseeing train and was mostly packed (apparently this is a national holiday), but the last car had a bench with a desk in front of a panoramic window. It wasn’t nearly as crowded.
It was only later that I found out that I was supposed to have a reservation for that train, but no one ever checked. It’s also another ¥200 to sit where I was sitting.