Abstract: An air-conditioning system in which a static inverter is driven at a predetermined constant frequency during a normal service mode of vehicle operation to effect operation of the vehicle cooling and/or heating unit such as to maintain the interior temperature of the vehicle within a chosen comfort zone. During initial start-up, when the interior temperature reaches extreme proportions due to the vehicle sitting in the sun or cold, an abnormal service mode of operation is called for in which the static converter is driven at an increased frequency and voltage for a predetermined limited time to rapidly bring the interior temperature within the comfort zone. The so-called abnormal service mode of operation may be indicated manually by the train service personnel under extreme temperature conditions after the start-up period of abnormal service mode has expired. Also provided is a passenger load sensing arrangement to increase the ventilation capacity of the cooling unit fans when overcrowding exists.