Let me preface this video by saying I don't endorse farming kills like this. I just went with the flow and didn't expect there to be another player that was helping fight off the zombies. That being said this is probably my personal best in kills so far playing the game. If and when I'm the last player alive, if I have an RPG in hand, I try to gather 5+ zombies together and use up all my RPG ammo accordingly. It is then I will push the train to the end.
The game recently updated and introduced a new zombie type named Conductor. He is essentially a tank. The conductor has increased health compared to the rest of the zombies. Most importantly he is invulnerable to knockback from all weapons. Upon the conductor hitting a human he explodes knocking back all players in his radius.