As of September 7th we can proudly say that Sofia is now equipped with a real and working underground railway. It is now connecting the east and west district of the Bulgarian capital. That means the two largest communist-pride style compounds Lyulin to the west and Mladost to the East. It is not a real matter of tourist interest though, since using the metropolitan you won’t get to see any of Sofia’s sights. Nevertheless if you’re trying to choose a cheaper place to stay, away from the center, but you still want to be close to the most important sights of town, you should bare in mind the possiblity of moving around underground. The newest station is at the Sofia University, close to all of the landmarks such as the - St. Alexander Nevsky cathedral, the Parliament, The Yellow Brick Road etc. Check out the map:
http://www.metropolitan.bg/bg/map/
Unfortunatelly in simple and understandable Bulgarian ss you can see. If you want to know some details on it, leave a comment.