This is by far the easiest and fastest way I found to do this and it's courtesy of xboxachievements.org.
Play sequence 5 mission 2, the one where you need to break Ned Wynert out of police custody. Once you reach part 2 of the mission, you're tasked with jumping onto a train to find engine parts. There are about 10-11 enemies on the train, so ignore the mission objectives and lure them onto the carriages to kick them off. Once everyone is dealt with, go to the engine at the front and detach the rest of the train. You will desynchronise when you travel too far away and restart back at the train again, but the game will register the kicks off the train. Repeat until the achievement unlocks (about 10-15mins tops depending on amount needed).