German high speed rail lines are also shared by passengers and freight, it is not problem. Old rail lines can almost never be used for high speeds, all high speed rail lines are built completely new. Original constructors never thought about speeds around 300 km/h. For example, HS line can afford slopes with steeper grades, because high speed train units have more powered axles and have less problems with them than one or two engines with 20 cars. On the other hand, you need curves with much higher diameters. So HS lines are different from the ordinary ones.