A tgv train is abput 12m^2 and moves around 80m/s Wave drag of 1/2*rho*v**3 is about 4 megawatts for air. Long sides will reattach flow somewhat even with a ba

A tgv train is abput 12m^2 and moves around 80m/s Wave drag of 1/2*rho*v**3 is about 4 megawatts for air. Long sides will reattach flow somewhat even with a bad shape. With steel on steel so 0.001 to 0.002 m * C_rr * v^2 is about 2.5 to 5 MW. At a full load maybe even 6 or 7MW. I have nfi how to do skin drag, but it's probably around the same order. Thus we conclude that once Cd is below 0.5ish other concerns start to become a lot more pressing. Namely tunnel shockwaves, passing suction, driver space and comfort, crash ratings, down force for the engine car, and so on. Even if it looks cool so 10% more people use it, that's a net efficiency win.