Well, a lot of the MMOs nowadays have a system in place where mobs "leash" back to their original location if you go far enough from it, sometimes even if you're continuing to damage or generate aggro on them in an attempt to keep them on you. If you wanted to dump a train on someone else in that kind of modern MMO, you'd have to find a person who's already within range of a large number of enemies instead of gathering them from all over the place.
Additionally, they might not even aggro anything in your train if you don't get them to tag the mobs with an AoE by timing it correctly while they're fighting something else. Basically, anti-griefing mechanics to prevent people from bothering other people quite as easily. Naturally, people still find ways to bother people if they want to though.