Took me like 7-8 times. I found a guide that said, use a shotty or Deagle and grenades. Well, I didnt have either. I had an M4, Beretta and a grenade, so I threw the grenade first, then I went and tried CQ.Sgt Slaughter on the train keeps messing me up. Hes wearing nothing but a half helmet and a green gear but hes as hard to kill as those bomb squad looking guys that you have to shoot their helmet off , then shoot them in the head.
Thing about CQ with this guy is that its not as bad as you would think. On your first engagement, Drake will do a hit and counter move, and move to the other side of the car while pushing fat boy away to the opposite side. At this point, you can pick up an additional Nade on the crate there. Throw it, watch him stumble, and then get into the closed doorway at that end of the car. Let him come to you and engage him again, and just start beating the shit out of it, while hitting triangle to avoid his melee attacks, eventually he will go down, like 30 secs or so.
This is the only part of the car where I was able engage him in a CQ battle, if you try engaging fat boy with any amount of room behind you, Drake will only engage, and then do a quick move and disengage.
Good luck man.