Unless it's noisy at or near the desired communication frequency, it is trivial to isolate the signal and completely mute out the noise.

In this example, at ~19 kHz, a properly configured high-pass filter could easily remove any human-talking sounds.

I'm not worried about humans talking, I'm worried about the ambient noise of the train/plane itself.

Agreed. This particular app starts to fail badly when the AC is on my current room. Airplanes are much, much, worse.