Trainspotters, politely referred to as “railway enthusiasts,” are a distinctly British subculture. Stalking platforms and overpasses, armed with scanners and high-powered binoculars, they seem to be an obsessive, lonesome lot. As they shiver in nasty weather, clutching logbooks and timetables, they beg the question: Why? This film’s earnest and self-effacing subjects explain their passion with good nature and aplomb. An abiding taste for “skullduggery” is suggested as a possible motive.