I used my Suica card today for Keio Line from Shinjuku. It worked. My employer pays for my commuting expenses. About a month ago, being a novice in TYO, I ended up buying a Suica (for JR) and the monthly pass for Tokyu Line. I should have paid for both JR and Tokyu to get just one card, Suica. So some reasons, I don't recall I saw an option on the Suica machine to let me choose other lines for the origin and destination stations when I purchased the monthly pass. I find it is convenient that a Suica card can be used as both a monthly pass and regular prepaid card.