The Quadrail series is a sequence of five (as of 2012) science fiction novels by Hugo Award-winning author Timothy Zahn.
Together with Bayta, an enigmatic woman the Spiders have assigned to help him, he researches and combats the Modhri and later its controller, the Shonkla’raa.
The story starts with former government agent, Frank Compton, meeting a young man who drops dead at his feet.
The Spiders explain to Compton their worries of a weapon of mass destruction, which may be able to bypass their Quadrail security.
Compton agrees to help, and is given a pass for the Quadrails and they assign him a traveling companion named Bayta, who has a strange talent for being telepathic in her communication to the Spiders.
Upon arriving home, Frank Compton is awaited in his apartment by an unknown girl, who asks his aid in saving her sister.
There he learns both girls are part of an experiment by the Chahwyn, Compton and Bayta's employers, to create friendlier, symbiotic version of the Modhran coral they have been fighting for the past year.
Frank and Bayta plan to travel to the Filian sector of the universe, to follow up on the Mohdri-infected Filiaelians they encountered in Odd Girl Out.
After their discovery in The Domino Pattern that the Shonkla’raa, the original masters of the Modhri, were in fact a special cast of Filiaelian, and that the Filiaeli have re-engineered their genetic structure, Frank and Bayta escort Terese German, who seems to be of special interest to their enemies, to Proteus Station, one of the main Filiaeli genetic research centers.
The Quadrail is a train system including multiple tracks, each consisting of four rails, wrapped inside of a tube.
The tube allows for faster-than-light transport through most of the known galaxy and includes the 12 Empires recognized in Zahn's Quadrail series.
The Coreline is the mechanism which accelerates the Quadrail train system through an unexplained physical phenomenon, although the passengers aren't aware of this (general speculation is that one of the four rails provides the motive power).