text: Both "Warren Street" underground nodes were edited in http://www.openstreetmap.org/changeset/40382113 .  Historically http://www.openstreetmap.org/node/1781669415/history was a railway=station , but was changed by the author of that changeset why they changed it.  I'd suggest you ask them there (I'm neither a Londoner nor an expert on how underground stations should be tagged, so can't really comment on what's "correct").