Answer:
I come with a train and go with a train and the train doesn't need me but can't go without me.
I dig out tiny caves and store gold and silver in them. I also build bridges of silver and make crowns of gold. Sooner or later everybody needs my help yet many people are afraid to let me help them.
I have two bodies joined together as one. When standing still I run and run.
I am an object which is used in baseball. During the day I can't be seen at all. What am I?
Everyone asks for me but yet everyone hates to face me. For someone I am agony for others I am relief. Who am I?
No matter how little or how much you use me, you change me every month.