A garbage collector that employs the train algorithm determines whether futile collections have been occurring or the number of trains has reached a maximum. When either situation occurs, it merges a plurality of existing trains into a single new train. If the collector detects futile collection, it merges the oldest train with one of the other trains. If the number of trains reaches a maximum, the collection so merges trains that the next train addition will not cause the number of trains to exceed the maximum.