My son is autistic and had to smell everything.
Your child's knowing so much about trains and buses......that too sounds like it could be typical of autism as well. Autistic kids do have obsessions.....for my son it's currently bugs, reptiles, and fishing.
How is his development? Is it equivilent to his peers or does he seem to be behind?
Does he talk? Did he lose any speech? Is his speech hard to understand?
Is he social or anti-social? If he's social, are his social skills age appropriate?
Does he over or under react to pain?
How does he do with change? Can he handle it or does it shut him down as a person?
Is he sensitive to noise, textures, certain kinds of clothes, and ect?
The questions I asked are all autism related.