What is the theme in The Great Railway Bazaar: By Train Through Asia by Paul Theroux?

From the beginning of the book, the author expresses the great joy he has in riding railroad trains. The strongest impression one gets from reading the book is the author's fascination with meeting people on the train. Paradoxically, the slow and broken down trains as in Southern India and Ceylon, and in South Vietnam, are the most fascinating trains to the author. He enjoys watching the scenery from the train, and already in France, after crossing the Channel, he is imagining the lives of people that he passes.