Tanks should not be pulling trains pre 50. Healers don't have the tools to handle them yet, especially SCH. The 50s dungeons is where the train pulls begin and some players forget or pay no mind to the capabilities of their healers.

In these cases it's important to remember that if you do nothing you will gain no enmity, and as crappy as this sounds, when you see the tank bite off more than he can chew, let him die. Your only alternative is to go down with the ship, because as soon as you do anything, the angry pack will come for you after the tank drops.

This isn't something to spite the tank, or teach him a lesson. It's looking after your own ass.