A freight car in the United States spends approximately two-thirds of its time stationary; a significant portion of this time in hump classification yards is spent in queues waiting to be inspected and classified into outbound trains. This paper describes the development of a model for these queues and the application of the Monte Carlo method to the analysis of these queues for a particular classification yard. An elementary form of feedback is included in the model. Using the model of yard operation, certain methods of improving yard operation are determined. Operations Research, ISSN 0030-364X, was published as Journal of the Operations Research Society of America from 1952 to 1955 under ISSN 0096-3984.