Google have today launched a new transit layer for Google Maps. The new layer is available for 50 cities around the world.
To switch on the layer go to the 'More' button and select the Transit tab. You can also turn on the layer by clicking on a public transit icon on the map.
The Google Latlong Blog says that,
"the objective of the Transit Layer is to overlay lines visually on Google Maps. Think of a virtual metro map on top of Google Maps."
The full list of cities is Belo Horizonte, Berlin, Bordeaux, Brasilia, Cairo, Capetown, Caracas, Chicago, Copenhagen, Dallas, Dortmund, Duisburg, Düsseldorf, Ekaterinburg, Essen, Frankfurt, Genoa, Guadalajara, Hamburg, Helsinki, Johannesburg, Kazan, Köln, Lille, Lisbon, London, Lyon, Madrid, Marseille, Medellin, Mexico City, Melbourne, Monterrey, Montreal, Munich, Naples, Nizhniy Novgorod, Oslo, Paris, Perth, Portland, Porto, Porto Alegre, Prague, Pretoria, Recife, Rennes, Rio de Janeiro, Samara, San Francisco, Santiago, Sao Paulo, Seattle, Strasbourg, Toulouse, Tunis, Vienna, Warsaw.
Via: Google LatLong: New ways to get around with the Transit Layer