The cover for Judgement at Proteus calls it the Quadrail finale which sums up the downturn of the series. What could have been a perfectly entertaining open ended series unnecessarily became an arc that strangled the life out of the premise of a detective using his wits to solve mysteries on a train that runs between the stars.
The first 100 pages of Judgement at Proteus is borderline unreadable if you haven’t read the previous Quadrail book and it still dances on the edge of being unreadable even if you have. Still Timothy Zahn eventually recovers and Proteus Station eventually becomes the setting for some of the same logical games and switcheroos influenced by 40’s and 50’s spy and detective movies as the rest of the series. But it’s only when Judgement at Proteus leaves the massive alien space station and goes back to the Quadrail that it picks up properly and gets back into the flow of murder on the interstellar express.
Still the arc drags Judgement at Proteus down and the series suffers from the need for constant new revelations. The Modhri, a menacing pod people enemy who lives as intelligent coral that can take over any body is replaced as the series foe by the Shonkra-La a genetically engineered variant of the Fillies, who were once the master/slave race that ruled/destroyed the galaxy. And their main weapon, a telepathic whistle that can take over the mind of any race, except humans, is weak.
The Shonkra-La are basically equine Nazis, and Zahn manages to sell the idea, and even manages to make me overlook that the enemy’s big telepathic weapon can be defeated through the simple expedient of earplugs. Still the Shonkra-La, like the Modhri, is an enemy who would rather spend time gloating and entrapping Frank Compton in complex conspiracies, and only later charges at him with all its minions. And Zahn milks a certain amount of pathos out of the Mohdri’s transformation from a parasitic to symbiotic entity.
There are logical and plot holes in the Judgement at Proteus that you can drive a Quadrail through and it’s disappointing that Zahn or his publishers chose to end the series instead of continuing it as an open ended series. This book and the last have both been weak due to the arc, but I still have fond memories of a series that began with Night Trail to Rigel and offered a dose of classic Science Fiction with Asimovian mystery solving.