The various aerodynamic drag phenomena: pressure, laminar boundary layer, turbulence...
Railway Aerodynamics

To take a step closer to the physics of these phenomena, let's look at the evolution of air speed, as seen by a sensor placed at mid-vehicle height along the track, during the passage of a train in an ideal environment (no crosswinds, flat track and straight line) (figure 17 ). It starts with a peak at the nose of the train, then the air speed progresses gently along the train in the so-called boundary layer, with some disturbance at bogies and gaps between vehicles, followed by a larger, wider peak at the tail of the train.