Stuart G. Yates 
In The Blood [EPUB ebook] 

Detective Simms of the Pinkerton Detective Agency receives orders for another mission. Desperadoes are taking advantage of the fledgling railroads that cut through the endless plains, and the Pinkertons are charged with protecting the valuable cargo.
After a powerful group sets their eyes on the scarcely guarded carriages, Simms begins to shadow the train robbers. But soon, he realizes that someone is hunting him instead.
One of them will end up six feet under… by the bullet.