Jul 09 - using "por qué" to ask where something is

Hello all

I have been told that to ask which platform a train leaves from or which door is the exit, you say: "¿Por qué anden sale el tren?" and ¿Por qué puerta sale?"

I don't understand this, how does it translate and would I use" por qué "to ask " which cup do I use?" for example. Please can someone clarify this.

thanks margaret