Where there is a physical place available, the relative pronoun, 'where' takes a preference over 'in which', although 'in which' may still mean the same thing as 'where' in such contexts. However, it is doubtful whether they both can be used interchangeably since such usages depend on the clauses that follow the pronoun. Even in this case, we might see that they cannot be used interchangeably in all cases. However, as far as GMAT is concerned, if you see a place, and the pronoun 'where' choose that combo, provided other things are intact.