Comparing the cross-section of modern high-speed trains to old train waggons, which had straight walls and were limited to speeds around 200 km/h, I notice that most of them have curved outer walls. The width of the cabin decreases with height (and at heights occupied by passengers) and this obviously reduces space and comfort inside the cabin. So my question is, what are the main benefits of this? I am thinking of:
lower cross section of the train, i.e. reduced aerodynamic drag.
lower center of gravity. This is not bad with strong side winds!
lower moment of inertia for rolling around the longitudinal axis between the wheels for driving with high speed trough curves.
So what are the main benefits of curved outer walls? Is a circular shape the best?