A carload is as much as a car can hold: a carload of children, a carload of boxes, a carload of shopping. People say it when they mean “a whole car full,” not an exact number, because cars come in different sizes. It is an easy, informal word. Though actually, in American English carload can also mean a railway wagon full of goods, a carload of coal, because there car often means a train car. In British English, for trains, people are more likely to say a wagonload. When someone says “a carload of” in ordinary talk, they usually mean a lot, packed in, and they mean a road car unless the rest of the sentence is clearly about trains.