In young autists an absorption in the works of the late, great Rev W Awdry is almost universal. George, my older autistic son, went through his Tank Engine phase, but failed to obsess on Thomas, James, Gordon or any of the main players. George favoured Douglas, who doesn't feature in many stories, never appears on T-shirts or in pasta shapes, and speaks in a Scottish dialect that induces lockjaw when read aloud. Douglas has an identical twin, Donald, but Donald was nowhere with George. He bored him to tears, too.
Some obsessions recur less frequently.
A young man I know has a fascination with Sir Paddy Ashdown, to whom he attributes superheroic skills. Obsessions can tip over into phobias, and an autistic phobia can be very controlling. Since all three feature prominently in the average primary school day, life with Andrew can be hazardous. Obsessions come and go according to mysterious and ungovernable laws. The worst I've heard of is a compulsion to inspect the wheels of moving cars.