About 178 cities around the world, in 56 countries, have subway systems. Dozens of airports also have underground train systems connecting concourses and terminals. Go to any of these cities, any of these airports, and ride the subway, and if you pay close attention you will notice that if two stations are at about the same elevation, there will almost always be a contour like what you see in the figure. The train departs from station A and goes down a short decline. Then the track might be completely level for some distance along the path from one station to the next. Then shortly before arriving at station B, the track goes up a short incline and comes to rest in station B.
Why is there always a decline at A and an incline at B? Why does the track dip down and come back up? Continue reading