Training is just a method of getting players to react to an environmental condition, ie - move from a specific spot or have to CC. Mobs can be pulled away. Most mobs also die exceedingly quickly in classic, assuming you've got the right classes. In order for someone to train effectively, other people need to be positioned in spots that make such a thing opportune, and be unable to react sufficiently. Smart players will learn how to work the situation.
Imagine if dungeons inherently had trains coded into the NPC AI, which is what should really happen anyway. Why should all of these creatures stand around doing nothing while they hear their friends yelling down the hall or when they can see enemies from 30 feet away? To beat the content, players would need to clear out entire areas, rather than just heading directly to specific NPC's.
Dungeons fights are supposed to be like this (with more magic stuffs happening) -
https://www.youtube.com/watch?v=kyevhryWKHk