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This is not my puzzle, and I don't remember the source, but I don't see it posted here yet, so...

Update: After reading some of the alternative answers, I realize this puzzle would be better with an elevated station, not a subway, so I have included both versions. Either version supports the accepted answer:

Original puzzle: 1) A subway station has three escalators running side-by-side. One escalator only descends from the street to the subway platform, and the other two escalators only ascend from the subway platform to the street.

Better puzzle: 2) An elevated train station has three escalators running side-by-side. One escalator only ascends from the street to the elevated platform, and the other two escalators only descend from the elevated platform to the street.

In either case: Why aren't any of the escalators rigged to switch directions, based on time of day, train schedules, etc?

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    $\begingroup$ This seems less like a puzzle and more like a genuine question $\endgroup$
    – Caius Jard
    Commented Jul 19, 2019 at 18:31

11 Answers 11

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Because people enter the station at random times, while people mostly leave right after a train arrives. Thus, the peak exiting traffic is higher than the peak entering traffic.

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    $\begingroup$ Why does this support the premise of the question then that it isn't contingent on train schedules? As the exit peak is predictable one escalator's direction could be switched as soon as it is done rather than leave two empty escalators running in that direction. Then switch back shortly before the next peak. $\endgroup$
    – Martin Smith
    Commented Jul 19, 2019 at 15:59
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    $\begingroup$ @MartinSmith Because switching the direction of an escalator takes a nontrivial amount of time, and no one can use the escalator while its being switched. Escalators that do switch directions usually only do so twice a day (e.g. up in the morning and down in the evening for a station in the city, and vice versa for a station in the suburbs), and change over during off-peak hours. $\endgroup$
    – 2012rcampion
    Commented Jul 19, 2019 at 18:27
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    $\begingroup$ @MartinSmith see paragraph 3 of this article, which seems to indicate that the question's strategy is followed in real life. See also this article that discusses elevator reversal more generally, and says that "while the escalators [of today] are capable of being reversed, it is not common practice to do so." $\endgroup$
    – 2012rcampion
    Commented Jul 19, 2019 at 18:34
  • $\begingroup$ Switching one of the escalators from exit to enter would only be necessary to accommodate surges or peaks in the entering traffic. Stations afflicted with such traffic patterns are rare. $\endgroup$
    – A. I. Breveleri
    Commented Jul 20, 2019 at 3:11
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Perhaps the subway station

Also has a slide, like this

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It could be that the subway also

has a set of stairs. It is easier to go down stairs than up, so the stairs handle more downwards than upwards traffic.

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The subway station leads to the departing terminal at an airport. Lots of people will be arriving at the station to catch a plane, but few people will depart the station

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  • $\begingroup$ and there's another station for the arrivals? $\endgroup$
    – Kate Gregory
    Commented Jul 19, 2019 at 16:52
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The same reason that the doors on most retail establishments (at least when they're hinged instead of sliding) open outwards instead of inwards. If there's an emergency, e.g. building is on fire, gas leak, etc., you want it to be as easy as possible to evacuate.

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Disclaimer: There may be some overlap with answers given earlier.
The question mentions that the escalators are not rigged to switch directions. Which means they can be intentionally switched off at times while there also may be unplanned outages.
As is commonly seen, especially when there is an odd number of escalators available, ascending ones are typically prioritized over descending ones. This helps in having more redundancy for ascending ones, because, in general (let's say for 90% of the commuters) it is easier to descend stairs (thanks to gravity) and so it OK to relatively have less(er) redundancy for descending ones.

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The escalators might be used for dictating the direction of traffic: the "exiting" escalators would be on both ends of the platform, while the "entering" one would be in the middle -- thus directing passengers to move in the direction of the platform ends. I've seen a system like this in old buses in Beijing, where people entered in one door and had to exit through another further down, effectively converting the bus into a one-way street.

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Exiters must be kept separate from enterers for some reason. The entry escalator runs to the central platform with access to both tracks, and the exit escalators run from platforms on the outsides of the tracks so you need two--one for each direction. There are walls in the station keeping the two groups separate. Why are enterers and exiters kept separate? Because TSA must screen their bags in a process which resembles separating recycling into bins in the early 90s. (One bin for paper, one bin for clothes, one bin for small personal electronics, one bin for laptops...)

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Very few riders take an escalator down or up immediately after taking another up or down. Also, to rig escalators down and up in synchrony would mean slowing down or speeding up ones or the others.

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Reason #0

The accepted answer argues why demand for escalators tends to be in exit direction and therefore there would be no desire to rig them to reverse.
However, that is only the beginning.

Reason #1

You can't reverse the directions automatically because it is unsafe for anyone currently on the escalator. You need to ensure no-one is currently riding the escalator, which normally requires manual intervention.
Example incident where 18 people were injured
Another example where one person was injured

Reason #2

It also causes wear and stresses in different places, reducing the life of the equipment. This paper discusses the complexity.

Reason #3

It will be confusing for people - especially people running for trains. Wayfinding in train stations is a difficult problem, and adding variances by time of day makes it more confusing. Tired commuters learn habits of where they need to go, and changing it causes confusion and people getting in each other's way.

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@JonMarkPerry has the "correct" answer. I've ridden urban trains in many cities around the world, and the use of 2 "out" escalators is necessitated by the fact that JonMark mentioned - an arriving train can deposit hundreds of people onto the platform simultaneously (more-or-less).

It's also worth noting that, from the perspective of the station operator, it's essential to get departing passengers off the platforms ASAP. Overcrowded platforms are a serious safety issue for obvious reasons.

From the passenger perspective, the big push is to get TO the platform before your train leaves - that's why you see more people running to get TO the platform, not so many running to get FROM the platform. So passengers are frustrated at having only one escalator TO the platform, but safety dictates that getting large numbers of people away FROM the platform quickly be given priority.

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  • $\begingroup$ Jon Mark Perry hasn't posted an answer here - I don't know which answer you're looking at. $\endgroup$
    – Rand al'Thor
    Commented Jul 20, 2019 at 20:05
  • $\begingroup$ Welcome to Puzzling! (Take the Tour!) Generally, answers should add something that hasn't already been said. If you are simply reiterating a point already made, then typically an upvote (once you have the necessary reputation) is the recommended way to go. Can you edit this answer to differentiate it somehow from the others already given? $\endgroup$
    – Rubio ♦
    Commented Jul 21, 2019 at 10:50

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