It allows the train to maintain a constant speed through the twists and turns of existing track.它使火车在通过现有轨道的弯曲处时得以保持恒定的车速。It is important to maintain a constant temperature inside the greenhouse.使温室内的温度保持恒定很重要。All variables except one must be held constant.除了一个变量以外，其他所有变量必须保持恒定。