It would be easy to under-appreciate this book. At first look, it's a classic mystery novel, with a former spy in the lead role, set on a train trip. It's even a bit predictable, in a science fiction plot of a scary alien race bent on galactic domination. What makes this a decent book is the way the author delivers it, and the plot twists along the way. While many science fiction enthusiasts would see this as just a variation on a theme, there are a number of interesting ideas thrown in along the way.
For one, the concept of a network of "hypertubes" or "wormholes" to travel the galaxy has been done before, but to put it in the context of a framework of trains is a neat twist.
For another, the necessary background and experience to be a successful spy in a galaxy filled with fifteen alien empires is neatly realized, and figures in to the realism and pacing of the story many times along the way. Alien empires aren't new, but this is well done.
Finally, he keeps some things about the protagonist hidden until the final quarter of the book, adding to the suspense.
I have read several other Timothy Zahn books, such as the Star Wars (TM) trilogy - Heir to the Empire.
I would say this one is competently done, worth reading once, but I doubt I'll read it again. The others were better, but this one had plenty of neat ideas.
