Hand in hand they got off the train and she felt that her decision to leave was the best decision she had ever made in her life.

hand 的意思是"手"。 in 在这里表示"在...之中"或"以某种方式"的状态。 hand in hand 是一个固定短语，字面意思是"手在手里面"，引申为"手牵手"或"亲密地一起行动"，常用来形容两个人关系亲密或行动一致。