Train Man by P. T. Deutermann

On a dark night, a bridge is blown to smithereens, thunderously plunging a one-hundred-car-train deep into the Mississippi River. In Washington, the FBI scrambles-sending Assistant Director Hush Hanson and agent Carolyn Lang to investigate the deadly act of domestic terror.
Hanson is a team player and killer marksman. Lang has an agenda of her own. By the time the two agents leave Washington, they are on a collision course with each other. And another bridge has exploded.
Now, the investigation is exploding into an inter-agency feud. The brass is after a terrorist cell, while Hanson and Land suspect a single man-the Train Man-is bringing down the bridges on by one. But as more death and destruction strike the river, on one can guess that far greater danger is looming. A top-secret, emergency shipment of unstable nuclear waste has been sent west by train. And when the nukes meets the river there will be no way across, no time to turn back, and almost no chance to stop the deadliest disaster of all...