The single metro/subway line is nothing compared to bigger cities, but don't forget the three urban railways and the hugely popular bus rapid transit (line 550)! These form the basis for a network of high-service trunk lines in this wide urban area of only 1.4 million inhabitants.
Here's a nice schematic of what the system will look like in 2016 when the subway extension west of Ruoholahti is complete, the lines 560 and 500 have started, and the M and I trains have expanded to become the airport trains V and P: http://www.flickr.com/photos/29682104@N04/9711135644/
