A statistical method of defining rational heat loads on railway air conditioning system with taking into account the current changeable heat loads corresponding to current climatic conditions on the route lines has been proposed. According to this method the rational designed heat load on refrigeration machine, matching current changeable climatic conditions on the route lines and providing efficient operation of refrigeration machine of air conditioning system with maximum (close maximum) refrigeration capacity production (refrigeration output) for definite period of operation (monthly, seasonal or annular period) is defined through statistical treatment of data sets of hourly refrigeration capacities corresponding to the current climatic conditions on the route lines by their summation during the operation period for various installed (designed) refrigeration capacities of machine.The method is based on the hypothesis of different rates of refrigeration capacity production increment for the period of operation with increasing the installed refrigeration capacity, that is revealed in slowing down the rate of refrigeration capacity production increment at over increased installed refrigeration capacity. Proceeding from this hypothesis the rational value of heat load on railway air conditioning system is chosen close to the value that corresponds to the maximum refrigeration capacity production for the period of operation. Such rational value of designed heat load on railway air conditioning system provides reduction of refrigeration machine capacity and its cost by $15\cdots 20$% as compared with traditional its designing for the maximum heat load. The operation of refrigeration machine in partial modes for enlarged installed refrigeration capacity chosen traditionally - for the maximum heat load needs application of expensive inventor compressors to control motor speed matching current changeable heat loads.