First of all it's old and it takes some adjustment to get back into the mindset of 1975. When Theroux traveled to Vietnam the war wasn't over (he was horrified to discover entrepreneurs setting up battlefield tours) and when he went from the Soviet Far East to Moscow, it was still the USSR. In fact, that last part of the trip, when he traveled from northern Japan to the USSR in the winter, was the worst part of the book and not only because it was an old story politically. Theroux was also tired of the trip and irritable. I began to wonder (because I think he did) why he'd planned this trip. I'd also recently read Ian Frazier's excellent Travels in Siberia which gave me far more insights into this area than did Theroux who never even got off the train. Of course he'd not have been free to travel in Siberia in the 1970ies as Frazier was in the early 2000s. Still he came off as sick of the trip and not in the least interested in where he was. No associations at all with places and Russian history, for example, not even Yekaterinburg where the Tsar died.
The book seems to have been written as he went because he arrived in London with six notebooks full of text. It does read as if he was writing “in the moment”. The journey—and the book—begins at Waterloo Station in London and he began talking about what he was seeing and the people he was traveling with right then—to underline the fact that English travelers were as odd and noteworthy as the Malays or Burmese. He traveled to Paris and then to Istanbul on what was left of the Oriental Express (not very luxurious at that time). The idea was to travel the great trains of Asia and of those he found the Indian trains the most comfortable for travel—clean sheets and good food. Frankly, I became a little bored with the details of the accommodations. I suppose I was more interested in the destinations while Theroux’s fascination was with the journey itself.
While I share Theroux's fascination with trains which he attributes to growing up near the tracks of a major train in Massachusetts (my fascination came from the train that passed right near my grandparents in Duluth—zillions of hoppers full of red iron ore and engineers who always waved at me), I don't really like books that are primarily train journeys where the writer just passes through and notes all the exotic stuff. I didn't much like The Old Patagonian Express either--in both cases because I wanted to know more than the odd tidbits. I also read Dark Star Safari (more of less the route of the Cape to Cairo railway that was the dream of Cecil Rhodes). The through railway didn't actually exist so he spent more time in the countries and he went back to Malawi where he's been a Peace Corps volunteer years before.
In short I don't know why I keep reading Theroux because he's not a favorite. I guess it's the places that interest me and I never get enough because when he arrives he’s soon off on the next train.