In the U-Bahn station Eve gets into an older multi-car train with yellow doors. When Bill stops Villanelle from getting on the train behind Eve it's a modern single interior train with black glass doors.
Oxygen tanks and carbon dioxide tanks have different valves, so Villianelle could not have attached an oxygen regulator to a carbon dioxide tank.
The agents follow Villanelle into the U-Bahn station at Friedrichstrasse. However, when they get to the platform, it says Weberwiese, which is on the other side of Berlin.