¿Maybe a desire to create some downforce to keep the train on its track? There also is a difference in velocity and in expected air pressure, airplanes don’t h

¿Maybe a desire to create some downforce to keep the train on its track? There also is a difference in velocity and in expected air pressure, airplanes don’t have to bother about not creating pressure waves for planes passing them at a few meters distance while flying in the reverse direction and airplanes are relatively much longer than air planes compared to their width. That means that a much larger fraction of the air resistance comes from the sides of the train.