I did not know Grand Central has this amazing architecture until now. One of the secrets of the train central (others were deepest city basement, roof clock access, etc. which I think requires authorization).
These whispering walls are of intentional design, apparently. Grand Central is not the only one with such. One person speaks at the corner of the domed shaped ceiling wall, the other could hear it at the opposite corner. Wish I had known this earlier and showed it to my dad and brother.
Of course, I would have to try this out myself as well.