Have you heard of Darius McCollum whose obsession with trains was quite similar to this boyís and he learned everything about the New York transit system. His obsession with trains has gotten him arrested multiple times for joyriding and he has spent a fair amount of time in prison and no time at all working in a train related occupation. You are quite wrong that these obsessions are not harmful and your neurodiversity perspective totally trivializes these problems. Darius has taken various medications to try to treat his obsessions with no effect. It is likely that he will continue to do this activity and be arrested and in and out of prisons for the rest of his life.Jonathan is correct that Obsessive Compulsive Disorder can be comorbid with (that is, can occur along with) autism. But OCD and autism are not always linked, and often people with autism have intense interests that are not compulsive. Over the years, for example, our son has gone through phases of intense interest in trains, certain television shows, certain books and authors, and so forth. Now that he is older, he's able to take those interests and build on them -- creating stories based on favorite characters, building models of his favorite robots, and even putting together presentations based on favorite animals.
The difficulty, of course, lies in a parent's ability to distinguish between interest and obsession. When a person must engage in his interest in the same way over and over again or experience great anxiety, there is cause for concern. When a person can engage in true conversation about the topic of interest, can share his belongings or interests easily, or can build on his area of interest, then it makes sense to encourage the interest with the expectation of positive outcomes.
Sometimes, special interests are a great source of opportunity. Sometimes they're a serious problem. What's your experience with autism and special interests?