I have a ticket Antwerpen/AMS/YUL with a Thalys ticket to Schiphol and then a flight to Montreal on KLM. It is now called AIRRAIL ticket (1 booking for all tickets being considered flights legs but operating in partnership with Thalys). I have read that you check-in your train ticket max 4h before train departure by collecting it at the international desk at central station. Now if I am at the train station 4h before, there are two trains before mine passing by. Can I jump in one of these Thalys or can I use any other train to go to Schiphol (cheap direct IC ticket at 19€ exists)? What means check-in here knowing that most of controls are paperless? Any specific process for AIRRAIL in Antwerpen?
One purpose of the check-in is to get a document or code allowing you to board the plane. Even if it is paperless, it's still a distinct step for most airlines. It is often possible to do it online without going to a desk but that's really up to the airline.
Note that Thalys is a train with mandatory seat reservation, it's not permitted to board a train if you do not have a specific seat assigned (in practice this is checked a lot more systematically in Paris than in Antwerpen but that's the principle). Only frequent travelers with Thalys Club membership or some premium ticket holders are allowed to board the previous or next train (“framing” trains) and even then they are supposed to approach a train guard to get a seat. So your boarding pass will only cover one specific Thalys train.
On the other hand, if you did get your boarding pass for the flight, it may still be possible to use an IC direct and just bypass the Thalys leg (see Can I skip my train leg booked with a KLM flight?). Or, as suggested in a comment, to ask the desk for a boarding pass covering an earlier train. But whatever you do, you have to comply with the airline's process to get a boarding pass, just ignoring check-in and showing up at the airport is asking for trouble.