这个理解不确切。
过去完成时是因为这段的意思中已经有过去时了或者现在完成时的事情了,而她没坐过火车这件事发生在那件事情之前,所以只能用过去完成时。
e.g.
She didn't know how to get a train ticket because she had never travelled on train.
She didn't know how to get a train ticket because she had never travelled on train before.
两句都没有问题,从句用had的原因是主句用了过去时了,而从句发生在主句之前。