The trian _on which SJE was traveling was late.
选C.
首先,这是一个定语从句,SJE was traveling 修饰前面的名词trian
其次,定语从句不用what引导,排除D,
最后,这句话的意思是说“SJE所坐的那辆火车晚点了”不是说火车是SJE旅游的地点,所以不用where,而是表示他坐在那火车上（travel on the train）,在从句中做状语,而A which 引导定语从句时不能做状语