Quadrail, book 1 : Night Train to Rigel
Timothy Zahn2005

It begins when a man delivers a message for former government agent Frank Compton--only to fall dead at his feet. The message is a summons from the Spiders, the exotic and mysterious creatures who run the Quadrail, an incredible transportation system connecting civilizations across the galaxy. The Spiders believe that someone or something is preparing to attack their entire network and the worlds it serves, by smuggling battleships through the Quadrail--something that should be impossible to do. Compton, with the aid of a beautiful but enigmatic agent of the Spiders, is their last hope.

Because nobody else has been able to find the elusive enemy who seeks to enslave the entire galaxy…and Earth is its next target.