This is a more difficult case. At first glance there is clearly "white out" of the left hemithorax, with a meniscus sign. This indicates the presence of a pleural effusion. Why then is the trachea (arrow) moved towards the side of the effusion?
The clue lies in the appearance of the left main bronchus, which is abruptly cut off (arrowhead), in this case due to a cancer.
There is both an effusion and lung collapse. The volume loss due to the collapse is greater than the volume of the effusion. The collapse is therefore dominant and the trachea is PULLED towards this side.
The next image shows the CT scan of the same patient.