To reiterate: With the notable exception of the Eurostar trains to/from England to the Continent, there is no check-in. No one who will hold you back from bringing the kitchen sink (or foldable bike) on board a train.
You can bring as much luggage as you like, as long as you can carry it on board everything will be fine.
Yes, there are rules on paper. But the only place where these are enforced (sporadically) are the Eurostar trains.