The guns on trains are the same type you see on a field – they shoot those yellow tracers – and are called "ack." Flak, called "puffy ack," is the little black cloud or puff you might see if near a field, town, city, factory, HQ, or CV. Trains are generally used to supply factories, cities, and HQs and will be within 10 miles of these facilities. But the flak does not come from the trains.