The only way that I can think of to try to minimise stress levels for this poor kitten is sedation for the 7 hours she will be alone in the luggage car if that would be possible.
Personally I don't agree with transporting animals as luggage in either a plane hold or a trains luggage carriage. The only reason I can see for taking any pet on a plane is if there is an ocean in the way and then I would only do so if the animal was allowed in the cabin with me/ whoever was traveling with said animal, otherwise finding a way of traveling via car would be the only method I would consider. One member on this forum did in fact deliberately drive across a large area of the US to minimise the stress for their Bengals! Cost is secondary to the animals welfare and there is no way I would ever sell a kitten to anyone who would allow him/her to be put in a luggage car or plane hold alone for any length of time let alone 7 hours. That level of stress could potentially trigger you kitten into developing FIP which is fatal
You keep saying that because you have bought your train ticket you will just do this anyway and tough luck for the kitten if it is terrified but I see no reason why you have to take the kitten, you were already going to visit your mother so it's not like this trip was specifically to collect your kitten. Why can't you just arrange something else for the kitten and develop a more thought out, kitten friendly plan that will actually be better for the kitten not your wallet? I do tend to agree with others that your attitude of 'no I won't spend more to make my new kitten as comfortable and stress free as possible' to be worrying, it gives the impression that you would also put money before your kittens welfare in other areas such as veterinary treatment.
Your friends may see nothing wrong with your idea, many people may think it's fine but the people on this forum are passionate about cats (and other animals) and unlike a lot of people they put their animals on an equal setting to themselves and care and respect their pets as living creatures not possessions. Very few members will condone allowing a tiny kitten to suffer for the sake of saving money, and no we are not all millionaires, we just do everything in our power to ensure that we can financially support our pets adequately even if this means sacrifices in other areas of our lives.