Maybe you could try A Train Story by Eric Hill.  It’s about a race between a freight train and a passenger train.
assitant
Are you looking for specific trains?  Any fictional trains, or all trains?
assistant
What is a good book for a 6 year old boy?
user
Which types of books are you looking for?  Or can you clarify what types of books your child likes?
assistant
The kid likes trains.
user