Rule of thumb Water Always have with you. I don't know Finland and the rules for dog travel on trains. But if you need to have him in a carrier, then line the carriers' bottom with some absorbent material, in case of potty accidents
Also you can buy a little water dish that is attached to the crate.
I would also try to get both the water and some kibble food he is used to in his breeders home to take with me.
Also for accidents if he is allowed to be not in crate, some absorbent towels, we have something here called Sham Woo, and I love them. They dry quite quickly and pick up lots of liquid
If he is not in a crate, here we strongly recommended harness and leads, not collars to walk your Yorkie with. You will need his measurements beforehand so you can likely, pick up a kitty harness that you can put on him, and select a leash to go with the harness.
If it is going to be really really hot on that train ride, take one of your towels and run under cold water, and dab on his paws, under his groins to help to cool him down.
Depending upon the health and weight of your Yorkie, we also recommend for young pups and pups 2lbs and under, some Nutrical to have on hand. You do need to understand what are the signs n symptoms of hypoglycemia, and how to intervene/help effectively. This is a supplement that helps to bring blood sugar levels up.
I am so sorry, that you lost your tickets and had to purchase them yet again.
Please when you get home safely post pics. And your English is very understandable!