Guns are too Quick, will be one of your first Achievements.
Soon as Billy saves Rebecca from the Leech Zombie in the Dining Car (2ndF), he will join your Team.
Switch Characters using
to Billy and equip his Knife. Push
to leave Rebecca in the Room and head downstairs.
The Passenger Car has a few slow Zombies lurking around for easy Knife Kills.
If you are playing on easy, it should only take a few attacks to Kill a Zombie.
Hope it helps