One of the great things about Washington, D.C. is the Metro system, a network of public transportation, much of it underground, that serves the District of Columbia and a growing area round it. One reaches a number of Metro stations by taking escalators deep down beneath the surface of the city. Some of these escalators, I am told, are among the tallest in the world.

Once you reach the appropriate track, the train you seek will come within only a few minutes, unless it is there already. The train platform is a remarkable place. Why? Because it is governed by a single reality: the coming and going of…