Train Travel

For the cities you listed, you don't need a rail pass. If you're going to be traveling non-stop between Paris to Amsterdam or Paris to Brussels, to can save money by purchasing advance tickets via the Thalys.com website. But for all the other journeys you can simply purchase tickets right before you board the train- no advance reservations are necessary or possible.

For the itinerary you've suggested, point to point tickets will probably be far cheaper than a rail pass. Rail passes are usually only cost-effective if you are travelling virtually every single day and over longer distances....even then, some of the advance booking deals for tickets will undercut a rail pass. In addition, you must still pay sometimes-expensive supplements with a rail pass, as on the Thalys train that runs between Brussels and Paris.
For train schedules, the best site to review almost any country in Europe is the German rail website.
To find point to point ticket prices and discounts, check Belgian national railways, the Dutch national railway system, the Thalys website, and the French national rail website. For the last one, set the country in which you will pick up your tickets to "Great Britain." This will just keep the website in English, and will have no bearing on how you will actually obtain your tickets.
Because Belgium and the Netherlands are so small, rail travel is more like a giant commuter network, so tickets aren't necessary (and most times not possible) in advance...you just head for the station, purchase the ticket, and jump aboard.
Your best bet for a good discount would be on the journey aboard the Thalys between Brussels and Paris. Just remember that, as with airlines, the cheaper the ticket, the more restrictions there are on exchangeability and refundability.