I am trying to travel from Amsterdam to Bruges, Belgium in December. What do I need to have in order to travel on the train?
Aside from a train ticket? If you are asking about COVID requirement, this might be helpful:
Much depends on where you live, where you have travelled in the weeks prior to your arrival, your vaccination status, etc.
December is over a week away. With everything changing regarding Covid and border crossings so fast any advice today is likely out of date before you arrive.
Check carefully shortly before you travel and be prepared for anything.