On the train I saw a student---I thought was your sister.

I thought是插入语,可删去不看,On the train I saw a student (who was your sister)。在火车上,我看到的学生是你姐姐或妹妹。括号里是一个定语从句,先行词a student 在定语从句中当主语,这个句子没有形式主语。形式主语通常是以it开头,形式主语it 作为形式主语的it并无实际语义,只是为满足语法上的需要,避免句子头重脚轻,它代替的是句子的逻辑主语,常见的概括起来会有如下情况：1。不定式作逻辑主语　　如：It is very kind of you to give me a hand in time。你及时帮助我,真是太好了。2。从句作逻辑主语　　如：It so happened that the tickets were sold out。不巧,票都卖光了。）