In the earliest days of the London Underground, trains used steam engines, which would collect steam and release it only at specified openings above tunnels, so

In the earliest days of the London Underground, trains used steam engines, which would collect steam and release it only at specified openings above tunnels, so as to not suffocate everyone. In posh areas, fake housefronts like this one: http://www.urban75.org/london/leinster.html http://www.urban75.org/london/leinster.html were built with a hole behind them, and trains would release steam as they passed under the hole. More such cases in Brooklyn and Paris: http://www.dailymail.co.uk/news/article-2388179/When-house-home-The-fake-townhouses-just-hidden-portals-underground-world.html http://www.dailymail.co.uk/news/article-2388179/When-house-h...