halfanhourhaspassedsincethetrainleft.一句中

任何句子都是要有主语的，很显然这个句子里"the train"才是。since前面是时间状语，left是谓语。

had been passing是过去完成继续时态及被动语态,是不适合上面用的。应该更正 :Half an hour was passed since the train had left.那就是说火车已离开先。才经有半小时之久矣。从句 since the train had left比主句 Half an hour was passed早一点而说。