Underground railway stations or mass transit stations are of the most importance as they are the main part of underground structures seen by the public.
In some countries or town, specific efforts have been made and have transformed underground stations in real artworks. It has been the case from the origin for the Moscow metro, as well as recently for the Toulouse metro where each station has been designed by a specific artist.
Nowadays, efforts to integrate different systems of transport have led to the construction of underground stations which connect different transportation modes, being built underground to free the surface for normal traffic.
For example, in the city of Madrid a new station has been built under the very center of the city, in the Puerta del Sol square, connecting the underground metro system with an underground railway line.
Some other stations of this kind link networks of underground metro with urban buses.