S.K.
My son is 7 and still loves his trains. If you go to Barnes and Noble online and search for Trains in Children's book, you will find a ton of them. Below are some of the non-Thomas books that we have/had with the top 5 on the list being among the most read of the list.
I Love Trains by Philemon Sturges (we read this almost every night for at least a year)
Trains: A Pop-Up Railroad Book by Robert Crowther (a very cool pop up book)
Two Little Trains by Margaret Wise Brown (another favorite at bedtime by the author of Goodnight Moon)
The Polar Express by Chris Van Allsburg
Chugga Chugga Choo Choo by Kevin Lewis
The Little Red Caboose by Marion Potter
Seymour Simon's Book of Trains
Ed Emberley's Drawing Book of Trucks and Trains by Ed Emberley (He may not be quite ready for this, but Ed Emberley does a good job showing how to draw things by breaking them down into basic shapes.)
Freight Trains by Donald Crews
Inside Freight Trains by Donald Crews
Train by John Coiley (not a story book, but has lots of pictures and info on trains)
Trains by Byron Barton
My son also likes looking through a magazine called Model Railroad. He loves looking at all of the different model trains and layouts. And there is a series of DVDs called "I Love Toy Trains" that was a favorite of his. It's videos of all kinds of model train layouts set to music.