Doubt with para and a
 

Hello. I've problems with these sentences:
Una carta para España. - The letter to Spain.
El tren a Madrid. - The train to Madrid.
My MX friend says that sound silly in Mexico. I'm confused, maybe that's correct in Spain Spanish, cos I'm learning castellano Spanish?

I'm not exactly sure what your question is. In many cases, "a" and "para" can be used somewhat interchangeably.

Voy a la casa.
Voy para la casa.

I agree with Tomísimo. In this case, "a" and "para" both imply a destination.