Passenger trains have a pre-determined route. As they proceed along this route, they stop at specific stations and then proceed to their final destination, by which time all passengers must disembark. This provides authors Strauss and Howe with the analogy to describe a continual and sequential progression of generations through American history. Passengers board the train as infants, proceeding to the next station of rising adulthood, then to the station of midlife and, finally, to the station of elderhood. The train completes its route when the elders have passed on. Behind this first train, another generation has boarded its own train, looking very different from the passengers of the train in front and from the next train to embark in another twenty-two years.

The hypothesis is clear: There are only four generational types—Idealist, Reactive, Civic and Adaptive, each with its own set of characteristics, behaviors, attitudes, and activities. These appear at approximately twenty-two year intervals, defining and re-defining the priorities of American society and participating in events in very specific ways. As four generations pass through time, a cycle is completed. America has completed four such cycles and is currently in its fifth and incomplete one. During each cycle, there are two social moments, a spiritual awakening, during which values are uprooted and revised, and a secular crisis, a ten-year period of upheaval having so far included a war. Based upon this generational march, Strauss and Howe are then able to make general predictions of the future track America will take as the current cycle completes itself.