If you are moving from Kumamoto to Beppu, or stopping on the way in Aso or Oita, you might want to consider riding the Aso Boy train.
The train usually runs on weekends and holidays otherwise the scheduled route is served by the Kyushu Odan Tokkyu train.
The Aso Boy is themed around the mascot Kuro, a black dog that you will find throughout the train.
There are four cars on this train. I would recommend trying to get a seat in Car 1 Row 1–3 in the Beppu direction, or in Car 4, Row 8–10 in the Kumamoto direction to have a view from the big front window. If you reserve the seat at the other end, you will ride the train backward and just get a view from the back of the train, though that is also not bad at all. Your second best option is Car 3, the most themed car with special seats as well as the counter where you can buy food and souvenirs and a kids playground. Well, the kids playground might also be a reason to not take a seat in Car 3!
Going back to the subject of food, they offer different snacks such as pudding as well as different drinks. They also sell bento box that you have to reserve on the official website once you have your ticket, you need to reserve the bento around a week in advance minimum.
As I was a bit late on booking the bento, I could not get one, but there was plenty of options at the Aso michinoeki (roadside rest area) located next to Aso station. Most of them feature red beef (akaushi), or horse meat. You might want to memorize the kanji for red beef 赤牛and horse 馬 if you do not want to make a mistake in your choice. I also had a cup of sake that I previously purchased earlier at a local brewery in Takamori. In the train I grabbed some yuzu flavored cake and a rum caramel flavored pudding (as the regular caramel pudding was sold out). You can also purchase different Kuro themed souvenirs.
As there is only a small number of limited express daily running on that line, if you happen to use it during, why not check to take the Aso Boy?
If you want to ride it, just make sure to make reservation as the train is reserved seat only.
We were very disappointed that none of the kids activities in the "Enjoyment" section were opened.
One highlight of this train was that the train master was attentive to young children and spent time teaching them origami, but other than this, the above-closed facilities brought disappointments that overshadowed the better parts of the experience.