>>149880601
Understand that autism largely boils down to being incapable of understanding the world, which causes them a great deal of stress. Trains are easy to understand. They have well-defined routes and timetables, and their schematics and model information are freely available and fully documented. Minute to minute, their movements are easy to predict: it follows the tracks. Even the turbulence is rhythmic. When things are stressful, autists can calm down by focusing on repetitive sensations, typically called "stimming". Often, this manifests as obvious physical actions that make the autist look like a freak: rocking their body or flapping their hands or making repetitive noises. Watching the wheels and connected machinery run endlessly acts as a natural and comparatively subtle form of sensory regulation.
You can apply a lot of that to Sonic, too. His running, jumping, and spindashing animations are all spinning circles, the game was popular enough to have a big pile of information available for autistic fixation, and there's the added bonus of Sonic helpfully telling them that he's "way past cool," so they take that at face value.