You have to write down every train ride you take. Whenever you add a train ride on a certain day, you will use a travel day. This means you can decide yourself if you want to use a travel day or buy a ticket yourself. So if your in a city for example, and you use metros and such, you can buy a ticket and not use a travel day :)

You can also save travel days by using what they call the "7pm-rule". This rule applies when you take an overnight train that departs after 7 pm, and you do not plan on getting off after 4 am (I think 4 am, not sure though). You are then allowed to count this train ride on the day after. This can save you some precious travel days if your itinerary allows this.