Left to Lapse by Blake Pierce

When a body turns up on a high-speed train passing through France, Germany and Italy—clearly the work of a serial killer—authorities wonder: whose jurisdiction is it?

FBI Special Agent Adele Sharp—triple agent of the U.S., France and Germany—is called in as the only one capable of maneuvering the layers of authority and of tapping her brilliant mind to stop the killer.
But as more victims turn up—on other trains, in other countries—the case grows increasingly complex. Can this all be the work of one serial killer?
And if so, where will he strike next?