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?
The answer is that by doing this track contour, the operator of the train system saves wear and tear on brake pads. When the time comes to slow down because station B is near, the slowing-down does not need to be accomplished entirely by means of brake pads. Part of the slowing-down is accomplished simply by going slightly uphill.
Of course with many modern-day trains the cars do regenerative braking and so this also reduces wear and tear on brake pads.
The other thing is that this track contour provides a modest energy savings because not quite so much energy has to be spent accelerating the train up to speed as it departs from a station on its way to the long level track that connects the stations. The fact of the train increasing its elevation while slowing down to approach station B is the hundred-years-ago pure-mechanical equivalent of using regenerative braking to convert kinetic energy into stored energy in a battery. The upward slope toward station B converts kinetic energy into potential energy, stored by the slightly higher elevation of the train. The saved-up potential energy is put into use the next time the train departs from a station and goes slightly downhill again.
So in return for a very modest one-time cost at the time of track construction, the operator of the train station has a gift that keeps on giving, saving energy and brake pads.
I have noticed this in the airport train system at the Denver airport, the Boston T system, the New York City subway, the Washington DC Metro, and the Barcelona Metro.
Had you already noticed this nearly universal aspect of subway track design prior to reading this blog post? If so, congratulations and you get nerd points!
If you notice this at some airport or city subway system, please post a comment below.