If you take prescription medication, then it's imperative to have it with you. Count out the days you'll be gone, then add one or two extra in case you extend your stay.
Multiply by the number of pills you have to take to figure out a total. Research the laws for the countries you're passing through and get an official letter from your doctor.
Being hungry while stuck on a train will spoil the entire mood, so be prepared with snacks. Onboard restaurant carriages are often expensive, so try to pack your own food.
To look and feel your best, bring a small toiletries bag with the essentials, such as a toothbrush, toothpaste, mouthwash, wet wipes, facial tissues, and hand sanitizer.
It's hard to predict how hot or cold a compartment will be, so dress in layers and be prepared for any situation. Bring slippers, scarves, and thin cardigans.