Asperger's in 8 Yr Old Son

Obsessions are common, but the school psychologist is unlikely to care unless the train obsession interferes with not only your son developing other hobby interests, but with learning and being able to hold at least short non-train related conversations.
If he also loves these other things, he doesn't have a monomania. Pop culture tends to overemphasize monomanias as a sign of Aspergers. Plus, I have a number of Aspie students whose monomanias do not fit the stereotype at all. One girl knits, another boy reads biographies, at least 3 like drawing anime.

My son. OMG. My son was into Thomas for 9 freaking years! I almost miss it now lol. He is 11, and last year he finally got out of thomas. He still plays with them occasionally, and I bought him a Flying Scotsman take along for Christmas for old times sake, but for the most part he is now fully integrated into minecraft. This seems to be a progression for a lot of aspies, and it is a more acceptable hobby for the age group, lots of kids are into it. Most not quite to the depth that my son is though. He also likes Sci fi, which a lot of aspergers kids do. Dr Who is his ultimate favorite.
It was really tough when he was a 9 and 10 year old and he was still into Thomas, my son knows every tiniest detail about his subject, and could and did talk for HOURS to anyone who showed the slightest interest. He knew though to keep it to himself at school.
He likes no form of sports or competition, stresses him out enormously. I have stopped trying to push him into anything like that.

Neil Young and his spec needs son were so into trains, he bought the Lionel Train Co. Well he was a major investor when it was last sold anyway. Here are some cool stories and pix.

http://www.feelnumb.com/2010/02/23/neil-youngs-obsession-...

http://thrasherswheat.org/tfa/trains-neil-young-1993.htm

He's also involved in a special needs school in CA.

http://www.bridgeschool.org/

Ok, yeah, and I'm a big fan.

:)