Each passenger can bring one carry-on luggage with max. dimensions of 55cm x 35cm x 25cm that'll fit under your seat, plus a small handbag with max. dimensions of 36cm x 27cm x 15cm.
Luggage with larger than allowed dimensions, or additional luggage, can be added at time of booking for a €5 fee.
Taking a bike on the train also requires extra baggage to be purchased.
The cost increases to €20 if you don't buy in advance and instead pay on the day at the station.