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southbound + noun
southbound 是形容词，常作定语修饰 train、traffic、lane 等名词。
The southbound lane is closed for repairs.
南行车道因施工关闭。
向南行的；向南行驶的
先理解核心含义，再看它在真实句子里的用法。
向南行的；向南行驶的
southbound
The southbound lane is closed for repairs.
南行车道因施工关闭。
正在(或将)向南行驶的。
Which is (or will be) travelling south.
The southbound train leaves at 9:15 from platform three.
南行列车于9点15分从三号站台出发。
Traffic on the southbound lane is moving slowly because of road works.
由于道路施工，南向车道上的车辆行驶缓慢。
We took the southbound bus to get to the beach.
我们乘坐南行公交车去海滩。
southbound 是形容词，常作定语修饰 train、traffic、lane 等名词。
The southbound lane is closed for repairs.
南行车道因施工关闭。
也可作表语表示“正朝南行驶”；如需说明具体路线，常接 on + 路名。
The ship was southbound when it lost power.
那艘船在失去动力时正向南航行。
Not this
The car drove southbound along the highway.
Use this
The car drove south along the highway.
southbound 是形容词，不能跟在 drove 这类动作动词后作方向状语；表示“朝南行驶”应使用副词 south（或 southward）。可以说 The car is southbound 表示状态。
Not this
The town is southbound of Beijing.
Use this
The town is south of Beijing.
southbound 没有介词用法，不能后接 of 表示“在……以南”；表示方位关系应使用 south of。
Not this
The southbound part of the island is warmer.
Use this
The southern part of the island is warmer.
southbound 只表示“向南行驶或延伸的”，不表示地理上的“南部的”；指位于南方的区域要用 southern。
Focus on the distinctions below. Other candidates are kept compact until a reliable usage note is available.
南行列车
The southbound train was delayed by an hour.
南行列车晚点了一小时。
南行交通
Southbound traffic is heavy this morning.
今天早上南行交通拥堵。
南向车道
Stay in the southbound lane until you reach the junction.
在到达交叉路口前请保持在南向车道行驶。
南行航班
The southbound flight took off on time.
南行航班准时起飞。
南行车道
The southbound carriageway is closed for repairs.
南行车道因维修而关闭。
Use southbound naturally, then get feedback on meaning, tone, grammar, and phrasing.
Revisit a common error or keep this word in your review list.
记法：south（南）+ bound（前往）→ 向南行，想象路标箭头指向南方。
语域：neutral 常见场景：交通报道、航班信息、天气预报 southbound 是中性表达，通常用于交通、航班和天气预报等场景，正式与非正式语境中都比较常见。
上述各词都以 -bound 结尾，末尾读 /baʊnd/。但它们与 southbound 并不完全押韵，因为 southbound 的重音在第一个音节上，而 -bound 不带重音。注意 south 中的 ou 读 /aʊ/，与 southern /ˈsʌðərn/ 不同；也不要将 bound 读成 bond /bɑːnd/。
Dictionary facts from ECDICT, WIKTIONARY.