It is not clear whether the restaurant is a building directly opposite the train station, or whether it's simply beside a large building opposite the train station. This is because we don't know exactly which building Peter was asking about when he described it as 'the building opposite the train station'. He could have meant, "Oh, do you mean the restaurant is the building opposite the train station?", or he could have meant, "Oh, do you mean the large building you just mentioned is the building opposite the train station?