According to wikipedia, modern passports and visa is something that got introduced after World War one.
passports and visas were not generally necessary for moving from one country to another. The relatively high speed and large movement of people traveling by train (the closest modern analogy would be commercial supersonic flights) would cause bottlenecks if a regular passport control were used. Passports and visas became usually necessary travel documents only since World War I.
Does this imply that before WW1 there was no concept of distinct european nationalities (e.g. Dutch, Belgian, German, French, etc) and that you were considered a citizen when you simply lived in a country.
If not how did people identify themselves and were there really no threshold imposed on crossing borders?