system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are subway staircases so windy?

Everyday on the way to work, my fellow commuters and I are always feebly trying to hold our shirts/dresses/hair etc. down as we walk up and down the subway stairs.  

What causes this?  Is it the air circulation system pumping in oxygen?  Is it the constant repetition of trains rushing by causing high and low pressure areas combined with the change from being above ground to below ground?
gpt
That wind is the subways operating in tunnels with little clearance and provide a "plunger" effect, drawing air in from the departure site and then expelling it at the destination.  It more or less pushes it along with the train creating positive pressure ahead of the train and a negative pressure (vacuum) behind the train, pulling air into the tunnel.

There are other factors too, such as the thermal gradient, which, to a 5 year old means air moves relative to the temperature, and it's always colder under ground, so with air cycling in from the outside, it's always mixing.  The effect is much greater when you have a limited access tunnel like a subway where one end is warm and in the sun, and the other end is cold in the shade.  

Lastly, the outside wind will find its way in at terminals and exit points.

Your guess was pretty good.

For reference, you'll have windy caves if there's more than one entrance that operate on the thermal and wind effects alone.