The piston effect Wikipedia page¹ is a good accompaniment for this, and has tonnes of links to rabbit hole yourself in.

I seem to recall there being some really intriguing TGV tunnel hoods with various suppressor designs to reduce the noise to surrounding villages, but I can't find them now. Anyone know of a good gallery or site for them?

¹ https://en.m.wikipedia.org/wiki/Piston_effect