A garbage collector collects a dynamically allocated heap by employing the train algorithm, in which "car" sections of a heap generation are organized in groups, or "trains." When a car section comes up for collection, objects that it contains are evacuated if they are referred to by references located in cars not currently being collected. The cars to which they are evacuated belong to the trains that contain the references. The trains form a sequence in which their constituent cars are to be collected, and objects that are directly allocated in the generation are placed into trains that precede some existing train in the collection sequence.