One blast of the whistle means "stop at once," or what is known as "down brakes"; two blasts of the whistle mean "off brakes"; three blasts of the whistle mean "back up"; a continuous blast means "danger." A semaphore signal at right angles to the post indicates danger; when the semaphore drops to an angle it is a signal to proceed. A red lantern indicates danger, as does a red flag; a green lantern or a green flag indicates "caution." Lanterns which are swung at right angles across the tracks mean "stop"; a lantern raised and lowered means "start"; when lanterns are swung in a circle it means "back the train."