In the American campaign at the end of the Air Raid Against Indochina mission, a cut scene will be shown with a train being loaded full of supplies. Strafe the train but leave it with a small amount of health. You can destroy the machine gun mount on the rear which will make this easier, especially on Veteran. Follow the train down the valley (destroying the 30 military installations along the way) and as it closes in on a tunnel it will pass by a refinery. Strafe the large fuel containers when the train is next to them and the resulting explosion will destroy the train and unlock Trainkiller.