walk along和walk down这两个短语都是“沿着.走路或散步”的意思,可以互换,意思上基本没有什么区别.但常见　walk along that（ street）,walk down that（ road）.可见搭配上还是有一点区别,另外,口语表达中更常用walk down.
walk up 有两个意思,一个是招呼人“快来看啊!”另一个是“走上前来,向.走过来”,比如有个人走过来和我说话,a strange walked up and talked to me.