The Second Assault assignments are quite glitchy so there might not be a clear answer as to what the game considers "inside the Metro", but it seems to be the lowest level running alongside the trains. You would think that the section up the stairs but still inside the main building would be included but it doesn't seem to be. I was near the elevators on the lower level when I completed the assignment (I killed an enemy who was near the door to the little side path leading upstairs). Also, there seems to be support for this from an EA employee at the EA answers forum.
I'm not sure if both you and your victim need to be on the lower level or if it's just you or just him, but your best bet is to fight on the lower level and go for targets that are also on the lower level.
Good luck.