Train Travel at Christmas time

I am wondering how full trains are during the holiday season. We are a family of 3 and have a 2nd class Eurail pass for a few weeks of travel. Do we need to book seats in advance? For instance, we plan to take the RJX from Munich to Salzburg on December 22nd. Do we need to reserve seats? We are not opposed to standing but is there a chance we will not be able to get on a full train or worse have to wait for another train at this time of year? Thank you in advance!