Simply load up campaign mission 18 "The Ghost Killer" and proceed through the level until you are making your way up the train. Once you have shot down the two helicopters and one crashes into the train, sprint forward to meet the enemies.
You will hear Hesh announce "They're roping up the sides!" and that is your cue to stop them.
I chose the right side and you will know if it registers correctly as the body will fall back down off the train.
You can restart from last checkpoint if you fail to get them in time.
I created this video to help:
Hope this helps.