Changing trains in Paris will often require a cross-city Metro ride. The precise route will vary depending on the stations involved. An up-to-date metro map can be found here.
Stations:
There are five principal mainline rail stations in Paris:
- Gare du Nord
- Gare de l'Est
- Gare Saint-Lazare
- Gare d'Austerlitz
- Gare de Lyon
The most common route is between Gare du Nord and Gare d'Austerlitz, as the Eurostar service terminates at Gare du Nord, whilst access to much of the south of Europe is reached through Gare d'Austerlitz.
The journey between these two stations only takes around half an hour, although you should allow plenty of time to make the journey, as it can be a little confusing if it's your first time using the metro. If travelling between Gare du Nord and Gare de l'Est, you can easily walk it in 10-15mins, although make sure you give yourself enough time, especially if carrying lots of luggage.
There may well be check-in time requirements for train services, although this generally only applies to Eurostar (check-in at least 30mins before departure).

