OS reviewed:

Alternatives: iPhone/iPad users can try the free app . After a predeparture download, MetrO will store information on more than 400 subway systems across the world.

aMetro allows you to download and view maps of public transit systems for 180 cities all over the world.

For additional information, such as which public transportation systems are covered in the map, you can click on the "Details" button. For example, the app's Moscow map includes information about its metro, train and waterbus (while the metro runs just within Moscow and its suburbs, the train takes you to other cities).

After you enter your station of departure and destination, the app estimates the shortest travel times. For London, Brussels, Moscow and St. Petersburg, the app will even find the station nearest to the user's location. It would be an additional plus if the app were synced with public transit to tell you when the next train will arrive.