Railroads? Nationalization of the US railroads during WWI was definitely a long-term loss for a short-term gain, although it was only ever meant to be temporary I guess.
Privatization of the railroads neutered a lot of their utility in much of the US. I remember taking my first Amtrak in Illinois to Chicago and experiencing that my train was delayed for 3 hours because the tracks were privately owned & slow freight trains had priority on the majority single track rails. I later learned that such delays were typical.
German rail has been absolutely awful since it was privatized.