He may be having an issue. Something jumped out at me in your post though that may get at a cause for his issues. How much does he like the trains? Could you describe how he plays with them, and does his enjoyment for trains include knowing information about them?
What other transitions does he have issues with? Does he have trouble at school moving from one activity to another? Does he prefer order and routine?
Does he enjoy the company of adults more than kids his own age? Does he try to "teach" you or tell children what to do while they are playing, like "you say this, then I will say that..." Does he ever recite dialoge from TV or movies?
Is he a picky eater, have trouble with texture or fabrics? How are his motor skills?
Is he very bright and verbal, knowing many words and definitions beyond where he should be developmentally?
If anything I have asked you promptes a response, you may want to see a Developmental Pediatrician and find out if he could be helped with some intervention so that he is more comfortable with tranisitions, including those to school.
M.