My paper reads Willa Cather's short story “The Affair at Grover Station” (1900) and an episode from My Ántonia (1918) alongside the White-Slave Traffic Act or “Mann Act” of 1910. Placing Cather's work in dialogue with Progressive Era constructions of sex trafficking, I intend to show how her fiction not only registers but also complicates the logic of her era's anxious imaginative and legislative response to increasingly mobile populations. In the early “Affair at Grover Station,” as in My Ántonia, Cather invests the prairie with a freedom of movement that can be positive as well as deadly. But another site of wide-ranging mobility typically marks her vision of the west: the railway. I argue that, for Cather, this site of mobility differs crucially from the prairie's elemental freedom in its manipulability. In a notable parallel to white-slave traffic narratives, the story and novel both feature villains who manipulate the railway system in order to entrap or assault young, unmarried women. My reading makes a key distinction between the legal and fictional texts: while the Mann Act quells an imagined form of immorality generated by the transportation of victims, Cather imagines mobile villains whose victims are static. However, bringing these texts together also reveals how similarly racialized threats allow both Progressive Era reformers and Cather's fiction to associate mobility with the endangering of a generation of young women.