"实际载货量" can be translated as "actual cargo load" or "actual payload capacity" in English, depending on the context.
"Actual cargo load" emphasizes the real amount of goods being carried at a given time.
"Actual payload capacity" might be more suitable when referring to the maximum weight or volume of goods that can be carried in a particular vehicle or container under real-world conditions, as opposed to a theoretical or rated capacity.
In most cases, "actual cargo load" would be a straightforward and commonly used expression.