Pretty much everyone who knows my kiddo knows his obsession with all things transportation-related. It started with Thomas the Tank Engine, which has some oddly mesmerizing hold over the toddler male demographic. Then we got into Pixar's Cars -- great timing for that one because now with the sequel coming out, we can't even buy cereal without Lightning McQueen and crew staring out at us from every shelf. And along the way we have accumulated other vehicles of various origination and type. Most days my living room looks like we are about to hold a public auto auction.
Byron Barton's Trains. It's a classic that I remember from my bookselling days as being steadily popular, along with Barton's other titles like Trucks, Planes, and Boats. Barton's illustration style features chunky yet remarkably detailed translations of the theme, loads of primary colors, and pleasantly appealing humans, similar to Todd Parr's, who don't get in the way of the action. In short, all the things toddler boys are drawn to.
If you can't find these in your library (and they are older -- Trains is copyrighted 1986), look for the board book versions online or at your local indie bookstore. A great addition to any little man's collection!