system
Given the following post, respond as if you're commenting on Reddit:
human
How does the air in a tunnel behave when a train passes through?

I ride the subway to work every day and I've been wondering about this. How does air in a tunnel behave when a train goes through it? Does it get pushed forward or backward compared to the direction the train is moving? Compared to the train's velocity, how far does the displaced air mass get pushed?
gpt
Cannot speak to the full science of this.  However:

In the Eurotunnel, the size of the tunnels (there are 3 - one big one each way and one service tunnel in the center) was expanded multiple times in the design phase.  The size of the trains was always expanded to basically completely fill the new tunnels.  So the trains were pushing huge columns of air in front of them.  This would obviously slow them down significantly.

To fix it, they dug vents between the two main tunnels.  When a train is running through the tunnel, the vents are dumping some of that air into the other tunnel. 

Of course, this means that if two trains pass each other, in different tunnels, with the vents open, they could derail each other because of the force of air coming out of those vents.  So they had to design a system that shuts the vents between the tunnels when the trains are nearing each other, then opens them again for efficiency when the danger has passed.

So, not an answer to the question (sorry!) but just to show that it is a real concern and design and engineering steps have to be taken.