she had never travelled on a train before,她之前从未坐过火车旅行.(新概念英语里)

she had never travelled on a train before,她之前从未坐过火车旅行.(新概念英语里)
1,根据前面用的had,首先在此句travelled应该是过去分词而不是过去式,
2,此句是什么时态?怎么知道?
3,可否去掉had,而 travelled看成是过去式,改成she never travelled on a train before?改了之後跟原句意思有什么不同?
请分开3个问题通俗易懂回答,