Labor spies, the furtive agents working undercover and reporting on workers in American factories and mines, have long fascinated labor historians. This essay finds their forerunners in nineteenth-century railway spotters, undercover agents posing as passengers who rode American trains and streetcars surreptitiously observing conductors. Railway spotting started out not to thwart worker militancy but to curb conductors' fare thefts, which were prolific. In the 1850s, aspiring detective Allan Pinkerton devised and marketed spotting to railroads and streetcar corporations eager to maximize fare collection in a nascent service-sector industry. Their robust craft solidarity enabled conductors to build strong unions, but fare theft was a solitary, secretive act not sanctioned in conductors' workplace moral economy. Thus conductors individually contended with spotters on their cars but proved unable to devise a collective strategy to combat them. Meanwhile Pinkerton sent a railway spotter to infiltrate the Molly Maguires in the Pennsylvania coalfields in the 1870s, and railway spotting spawned labor spying. Spotting led conductors to mistrust their passengers, but labor spies, in the guise of workers, bred suspicion and undermined solidarity among workers. Labor espionage quickly spread across American industry, while fare collection technologies gradually replaced spotters. Historians have treated labor espionage as a straightforward example of employers' drive for workplace control, but the spotter phenomenon shows how a repressive management technique grew out of a more nuanced negotiation between workers and employers of a complex workplace problem.