One bag, seven kilos. Two shirts that dry overnight, one warm layer, a rain shell, and a power strip with a long cord. Everything else was bought on the way and left behind.
A lower berth is worth the extra money on routes longer than eight hours. Skip the dining car on the first evening and bring your own tea. Arrive forty minutes early at stations you do not know.
Download the region before leaving the hotel. Keep tickets as files, not as links. A paper copy of the first address has saved more evenings than any app.