By 1924, just twenty years after the initial opening of the IRT line, the subway had more than quadrupled its number of stations. The original route of the IRT which opened in 1904 with just 28 stations in Manhattan. This 1924 map shows the extent of development the IRT had seen a mere twenty years later.

Yes, subways were quite well-established in major global cities by the 1920s, having been a revolutionary part of urban planning since the late 19th century. London’s "Tube" was the pioneer, having opened its first underground stretch in 1863. By the 1920s, the London Underground was a sprawling network. New York City opened its first official subway line in 1904, and by the "Roaring Twenties," the city was in the midst of a massive expansion under the "Dual Contracts" which built many of the lines still used today. Paris had its "Métro" (opened in 1900), and Madrid opened its first line in 1919. In the 1920s, subways were the primary way millions of urban workers commuted, helping to create the modern "vertical city" by allowing people to live further away from their workplaces. They were often characterized by ornate tile work and, in some cities, separate classes of travel that have since been abolished.