SWINDLED! : THE 1906 JOURNAL OF FITZ MORGAN by Bill H. Doyle

Fitz is ready for an adventure of a lifetime. Since his mother and brother died, his father has been reluctant to let Fitz out of his site. But 14-year-old Fitz thinks he's more than old enough to travel cross country to visit his aunt in San Francisco. So he boards the train with his trusty dog disguised as a baby, and sets off. Now one thing you need to know about Fitz is that he wants to be a detective. It's all he thinks about. And when he meets Justine Pinkerton, it's only natural that they become friends. After all, the Pinkerton family is just about the best detective agency ever. When a passenger on the train is poisoned, Fitz and Justine set out to solve the murder.