Underground Railways was a theme well represented throughout Railway Wonders of the World. Here is a complete list of articles covering underground railways, not covered by other topics, with links to the full articles.

Underground Railways

1.  Railways Under London
2.  Driverless Subway Trains
3.  A Scottish District Subway
4.  Underground and Overhead Lines
5.  Moscow’s Underground
6.  Power for London’s Transport  
7.  Chicago’s Unique Underground
8.  The Underground Electric