Hi Lilly,
Yes, you can add your journey on the day of the journey as long as you add it before you board the train. 
If you miss a train you can hop on to the next one, if it's still on the same day. Just make sure to add the journey of train you are planning to take. If you had a reservation for the train you missed, you'll need to go to the ticket office to let your reservation be changed so that it is valid for the next train you are taking.