it's because we have an integrated bus system with the metropolitan region.

So, you only pay one pass and u can go for anywhere in Curitiba or other city that is integrated.

And the "tube" is a easy and fast way to enter in the bus.

This video show the public transport evolution in Curitiba.

http://www.youtube.com/watch?v=4MtD3NGTMN0&feature=related

Regards.