What is also interesting is how the term "stacking" is used by different parks.
For example, at Disney World, the term "stacking" means holding lines outside of an attraction, even if there is space inside the attraction. The reason for this is to discourage people from lining-up for what they perceive to be a long wait, thereby distributing the crowd to other, less-busy attractions.
Since nearly all of CP's rides are on the outside, line-stacking would fool nobody (except, maybe, at Mean Streak, where the line is slightly more hidden from general view), so "train-stacking" is used, with, I suppose, much the same result.
The only ride where I ever observed Disney-style stacking was the DT, and, judging by opinions posted in CP Place, that won't work much longer, because nobody will believe there are that many people waiting inside.