I found on incarail.com website that:
In addition, that large baggage is not permitted on the train service. We recommend that you take the necessary precautions and carry equipment baggage not exceeding 8 kg (17.6 pounds), and of the same dimensions as permitted airline hand luggage.
How strict those rules are? I'm going to board the train with standard carry-on bag and city-backpack. Does anyone knows if its okay?