The terrorist boards Metro line 4 in station Montparnasse. He never changes line. He is pulled out of it in a station where you can see, at the front of the train, a passage going towards line 11. Being in line 4 to change to line 11, you have to take the train northward. But nowhere on line 4 northward, there is any such passage at the front of the train. The only such passage would be at the back of the train, in station Châtelet.