A great invention usually appears not only because someone thinks it up, but also because its time has come - some combination of historical, technological, or social conditions have come together to create a catalyst for the successful realization of an innovative idea. The steam locomotive, for instance, not only played a giant role in the Industrial Revolution, but was first inspired by it: a great leap forward in efficient land transportation was needed and sought after for countless applications. Again, the electric streetcar was born in the 1880's in part because the recent harnessing of electricity converged with both the explosive growth of American cities and a devastating animal disease, called The Great Epizootic, that had slaughtered thousands of horses throughout the Eastern United States during the previous decade.