أعط أجوبة - Русский язык

I'm trying to understand the difference between "в" and "на". So far I think I get the idea of when to use which. I though "в" was for countries, so I want to know why we say "Мы едем в Петербург" but we also say "Поезд на Петербург".

Sorry for the question not being in Russian, but I'm still too much of a beginner to word this whole question in Russian.

Thank you!