Yes- trying to retrofit the NYC system would probably run into the billion dollar range, because you'd not only have to modify every station, but you'd have to replace all the trains so that they fit the station gates. And the stations are not standard sizes - some have higher ceilings, some lower, and the platforms are different sizes as well. Parts of the system date back to 1885, and the main underground network opened in 1904. So you're dealing with well over a century's worth of construction styles and evolving design standards.