I've been touring in northern France, Belgium and the Netherlands for a few weeks. I'm in the process of taking the trains back to Paris for my flight home. I thought I'd share what I've learned about getting bikes on trains, because it isn't at all obvious.

Netherlands:
I can't comment on getting a bike into Amsterdam Centraal station. That place is incredibly busy, with tens of thousands of bikes parked outside all the time.

I did manage to get on a train at Rotterdam Centraal. The NS web site says that bikes have to be boxed. It turns out that you can roll onto the train in cars marked with a bicycle stencil, as long as you have purchased a bicycle day pass. You can't take a bike during peak times (before 9am, and between 3 and 6pm) or on the Thalys high speed trains. My advise is to simply ask at the ticket counter or information desk, because they know which trains have bicycle areas, and are very helpful with getting the correct routing.

Belgium:
I got onto a Belgian train in Roosendaal, Netherlands and tried to follow the Dutch procedures. That earned me a lecture from the conductor. In Belgium, you find a conductor on the platform and they put your bike in a special car (depending on your destination.) You have to buy a day pass for your bike separately from your own ticket, and attach it to the bike before boarding. Your luggage goes on the train with you.

France:
Bikes are permitted on the regional (non high-speed) lines during non-peak hours. They used to be prohibited on the TGV high speed trains, but now there are special areas for bikes on some trains. I couldn't get this information on the SNCF web site, so I asked the ticket agent at the station. There is a surcharge for the bike that is added to your travel ticket.

For TGV trains, the seats and bike places are reserved. I bought my ticket to Paris for tomorrow when I arrived in Lille today. I'll update this thread if and when I get back to Paris :-)

Notes:
It's hard to find up-to-date information on transporting bikes on the web sites of any of the national carriers I've used. I've had to resort to looking plaintive when asking for advice at the stations. All of the railway employees I've interacted with have handled my poor French and non-existent Dutch with grace, and spoken excellent English when requested.

I deliberately set up my routing to avoid the 'International' routes because the situation is very different for those.

The best advice I can give is to show up at the station early - but not before 9am - and politely ask for help. If you're not in a hurry, it will work out fine.