Never heard about pow camps in U-bahn tunnels.
@Spektre, If you find out anything about your grandfather's experience in France, I would be very interested. Especially if he has been on the pilotline to Spain. This line was an international affair, running from The Netherlands through Belgium and France, all natonalities taking over in their respctive country. My grandfather has been working on this in The Netherlands. I collected quite a few stories of pilots following this path to freedom and would like to know if your grandfather has been on this one as well.