I had this out of the Library for months, my son, barely a year at the time loved the pictures, above all. We bought this recently, apparently for 4 and above, my son, now 20 months has this book as a regular favorite for bed time. He loves the amazing illustrations of the train (I make train noises to go) and points to all the animals as we go through. I only have to say 'oi!, get off our train!' and he's found the book and demanding another read through.
As for political messages, John Burningham doesn't lay it on heavy but tells it how it is but in a way that kids can take it in without feeling they are somehow to blame. More and more material aimed at kids makes them aware of the state we are leaving the planet. As a child of the 70's I thought then, by now we would have it better sorted but there's still a very long way to go. I hope the next generation do better than we are.