If you prefer mysteries to be more fun than frightening, you'll be on the right track if you read Steve Hockensmith's On the Wrong Track (2007). The second book in his Holmes on the Range series makes amusing reading, while at the same time presenting a compelling, fast-moving murder mystery.
Old Red and Big Red, actually Gustav and Otto Amlingmeyer, are a couple of aspiring detectives in the Old West. More accurately, Old Red (so-called simply because he is the eldest of the red-headed brothers, while Big Red is the tallest) is the aspiring detective, a devotee of Sherlock Holmes. Big Red plays his Watson, the guy who goes along, provides some muscle and then writes about the adventures afterward.
This time the brothers get jobs on the Pacific Express, a train heading West. The Southern Pacific Railroad has been plagued by a gang of outlaws known as the Give-'em Hell Boys, and the railroad wants extra protection. Of course, the gang shows up, but there appears to be a conspirator and, indeed, a murderer aboard the train. Old Red, who happens to get motion sickness on trains, is determined to solve the case anyway. As for Big Red, he seems more interested in protecting a certain young woman, who may not be quite whom she pretends to be.
Hockensmith writes some amusing lines and throws in some slapstick situations, but there's plenty of suspense, too.