"Training" was a planned campaign mission in Call of Duty 4: Modern Warfare. All that proves its existence is a file called "compass_map_training.iwi" present in the game's source files. In the mod tools, in the "raw/English/localizedstrings" folder, a file called "training.str" includes information about this mission. The mission is most likely replaced by the SAS training mission. It was cut earlier in the development, as the extended SAS training (which was cut and replaced by a shortened version, probably in the last moment, as dialogs and objectives have just been cut, and the places where those objectives are completed, still are in the final training) contains elements from this one, like blowing up a car.