The Plum Grove Robbery Case

The Plum Grove Robbery Case (short story, 6800 words):

Gwen Conner is back in this new story. She's asked by the younger sister of a man accused of train robbery to help her brother. The sister insists her brother would never rob a train. Gwen learns that the take from the robbery was small, and passengers on the train weren't questioned by police. What will be revealed when Gwen untangles the loose threads of this mystery?