The problem lies in NM spawning - they are all but guaranteed to spawn and spawn in the exact same spot every time. If the spawn location of an NM changed, or spawns were delayed, there would be too much going on for 144 people to stay together.
AFK'ers no longer can rely on safety in numbers because there would be too much group movement. If spawns are delayed, it'd be most rewarding to work on as many NM's as possible concurrently - allows exp from mobs. NM's die slower because 100 people wont be in the blue area when it spawns.
As it stands now players have little option but to follow the train. You can go spawn your own NM sure. It just means the train will spawn/kill 2 to 3 NM's while you spawn one, which then the train will just come nuke the NM you finally spawn anyways. Break up the ability to predict what/when things happen, and people can no longer run the train.