Alex loved Thomas the Tank Engine when he was smaller and would often play with them for hours. If he wasn't playing with his own trains, he would be watching the videos/DVDs over and over again. He was never really interested in any other children's characters for long and would always come back to Thomas especially when upset about something. His Thomas trains were kind of a security blanket for him.
I have only just found out today (only about 8 years after the fact) that a fascination with Thomas (bordering on an obsession) is very common in children with Autism Spectrum Disorder. I found some information on this site about this interesting fact.
The things you don't realise until much later!