In a garbage collector that more efficiently places newly promoted objects into a generation managed by the train algorithm, a promotion train is established near the oldest train and the objects are placed therein. If some objects are referenced from existing trains in the generation those objects are placed into those trains, and if any such objects are referenced from several existing trains, the objects are placed at the end of the youngest referencing train. The promotion train may be a new train that is placed, or a existing train that is selected, between the oldest train and the youngest train at a position based on the amount of memory compared to the size of a collection set. In the case of multiple collector threads, multiple cars in a promotion train or multiple promotion trains may be formed.