I had occasion to ride the DC metro this past week, and noticed that many of the tunnels -- or at least the subway station tunnels -- appear to be elliptical in cross-section. The DC subway tunnels are particularly deep, at least compared with NY & Boston. I seem to recall that one goal was to provide some cover in case of nuclear attack. In any case, these tunnels are most likely under the water table, so that simply keeping their elliptical shape requires some effort. In the subway stations, the opposing train tracks are closer together than the foci of the ellipse. Indeed, the foci appear to be right in the middle of the passenger boarding area. This means that the "whispering gallery" effect provides cross-talk between the passengers waiting on either side of the subway tunnel, and means that the noise level is much higher than it would otherwise be. While the coffering of the elliptical surface does cut down on some of the noise, it is still nearly impossible to hear any announcements from the public speaker system. I guess there is no hope of reducing the noise level without completely covering the elliptical surface with sound deadening materials. On the plus side, I did notice that when 2 DC trains passed on another on opposite tracks, I didn't get the "high air pressure"/"low air pressure" sequence that I've noticed in the Boston subway.