When you're with Big Smoke for the mission Wrong Side Of The Tracks here's some things that'll make it easier for you. First put in the 3rd weapon set cheat (R1, R2, L1, R2, left, down, right, up, left, down, down, down) then go to your 3rd gun in the set. Now when Vegas gang are on the train, don't chase the train, go far in front of it and wait until the train comes. When its comes close lock on each of the people and shoot and kill them. When the first car past get back on your dirtbike and do the same thing 3-4 times and you should have killed all of them. But you'll have to be quick or else the train is going to past you and you'll fail.