RR
US
(TRAIN)
铁路，铁道（railroad的缩写）
written abbreviation for railroad US
(POST)
乡村邮件投递路线（用于美国乡村地区通信地址中，rural route的缩写）
written abbreviation for rural route: used in addresses in some areas in the US
(BREATHING)
medical specialized呼吸频率（respiratory rate的缩写）
written abbreviation for respiratory rate (= the rate at which someone breathes in and out)