Thanks very much for posting that - and don't I feel dumb!   
 
Despite the fact I have played the map loads of times, I have never noticed the way in to the room to get the blue access card.  That's a well hidden passageway!  Obviously, without that, I couldn't activate the barriers in the subway station.  I'd searched all over the station and in nearby areas - I really did, over and over -  but I simply hadn't spotted that passage hidden in the dark corner in the sewer. Doh!
I've got it now.  Thanks again. 
