Cross-border tickets are hard to find, and even harder to book. The Dutch railways' international booking site will often generate errors and even when you found the right fare, you might be confronted with a message that the seat is no longer available half way through the booking process - the search isn't made live, so you search in an inventory several hours old.
One of the better ways to book flawlessly is to either consult a specialised travel agency such as treinreiswinkel.nl in the Netherlands, or use a site such as loco2.com. These somehow seem to be able to circumvent the notoriously poor booking engines offered by national railway companies.
A few more travel hacking tips for acquiring cheap train tickets:
- Book early. Railway companies copied the yield management trick from airlines, so they offer a few cheap seats on most (high speed) trains. Make sure you know the exact start of the booking window, put it in your agenda, and make reservations on the first day of that window. It can be difficult to establish the booking window though. Sometimes sites such as loco2.com can help you by sending notifications.
- Find deals. Most railway companies have special offers for youth, elderly people, weekend fares, 'travel together' fares etcetera.
- For people from the Netherlands travelling to Belgium, try the nmbs.be website and look for the weekend fare from Roosendaal. It's not so attractive anymore because the fast trains run via Breda, for which this option is not available, but the weekend return from Roosendaal to Antwerp is only 9,60 euro (Brussels 16,60). These fares do not have restricted availability. For trains between Breda and Antwerp/Brussels the cheapest fares are currently 14 and 28 euros.
- Thalys, Eurostars and the other German and French high speed trains generally have low cost options with (very) limited availability. The cheapest train ticket between Amsterdam and London is approximately 110 euro with the added benefit of not having to pay for the expensive trains from London airports to the city center.
UPDATE 25/8. A colleague pointed me to oui.sncf, obviously run by the French national railways, but featuring more destinations and a more friendly user interface. It also includes a price calendar allowing for low fare search.