The Cat Who Blew the Whistle: Lillian Jackson Braun

This book was centered around trains and murder. A wealthy man obsessed with trains mysteriously disappears and everyone thinks he ran off with a younger woman after embezzeling money. Qwill gets the help of an extremely happy friend who Qwill believes giggles too much to help with the task. In the End Qwill and Koko realize the young woman actually murdered the man and took the money. Everyone seems to forgive and move on with their lives.