The Domino Pattern

The Domino Pattern is the fourth installment in the series. The Domino Pattern was published by Tor Books and was released in hardcover on January 5, 2010.

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. To get there they need to take one of the Quadrail super-express trains, which won't make any stops for six weeks. Once their journey has started, passengers start to die, apparently murdered, despite the fact that Spider procedures should prevent any weapons or poisons from entering the train. What follows is a race against the clock to find the murderer. 
(с) wikipedia