We advise you to start making reservations at least 2 months in advance. Many high-speed and night trains can already be reserved 3 months in advance, and will likely get fully booked sometime before its departure date (especially in busy travel seasons, such as summer and Christmas). You can check the train pages for more information on specific train services.
If a train is fully booked, use the planner in the app to find another train. To find alternatives that don’t require reservations, you can toggle on the option ‘No reservation needed’ in the planner.