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When you say "right under my apartment", where exactly do you mean? Because I also have a train line going underground very near my apartment but it's not directly under. Could I capture such images? And I'm on the 4th floor.

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I got confused at first too. What he means is "somewhere outside on the ground level while my apt is not on the ground level, there are trains passing by which I can see from my apt". You need line of sight.

This is not mysterious tech deriving images from sound traveling through the floor. You will be out of luck with your underground subway.

If you have the schedule and the sound I'm sure you could make a cam with translucent ground. You should be able to figure out where it is, which model and how long it is. Who knows, maybe the orchestra of break sounds (it has many) is unique enough to spot which it is exactly.

A fun fact is that there is a monorail station built in the middle of a residential building, so it might also be literal.

https://en.m.wikipedia.org/wiki/Liziba_station

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