Amsterdam to Brussels train

It will depend on your choice of train services, explained here:
https://www.seat61.com/trains-and-routes/amsterdam-to-brussels-by-train.htm

Thank you, so it depends if we want to save time (and $) we need to commit well in advance. If we want to wait and see, may be looking at a slower train.

not exactly how I would have compared the Thalys and the International Train.

I haven't recently read what Seat61 says, but here's my take...

Thalys is a fast(er) train, but all trains in the Netherlands are limited to 160km/h (and only 130 in most places) because of the sandy substrate.

There are two ways a train can be faster than its competition, overall speed and number of stops, assuming both are on the same route.

Thalys has both advantages, it can go a little faster in Netherlands, and significantly faster in Belgium than the International, and it stops fewer times at intermediate stations.

In exchange, Thalys is expensive and all seats are reserved, and sold by capacity control algorithms which try to extract the maximum money from each passenger. The only way to have low(er) prices on Thalys is to purchase far enough in advance.

The International train also uses the "high speed" line between Amsterdam Centraal and Rotterdam / Roosendaal but more slowly, and does not use the very high speed line in Belgium. So it is a little slower. But it is also a walk-up fare, anybody with a ticket can get on, and the price does not change. So it can be much cheaper, and you can choose what time and day to go right before travel.

So the balance is, quickest overall speed vs flexibility vs price vs comfort (Thalys a bit more comfortable but the train is weird shaped). You need to decide on each of those variables and decide which combination works best for you in your circumstances. There is no right or wrong answer.

As showed in the above article of Man in seat61 in case you travel with a slower IC (Intercity) train you can hop-off and on as much as you like as long you don't back track, use the shortest obvious route and during the day the ticket is valid. So you can visit Leiden, The Hague, Delft, Rotterdam, Dordrecht, Antwerp and Mechelen on the way to Brussels. All the stations there have luggage lockers, however not so sure about Mechelen. Dordrecht is not only beautiful but also the place to take the waterbus to the windmills of Kinderdijk if that is of interest.

If you buy a ticket Amsterdam to Brussels for the slower IC train, the train will call only at Schiphol Airport, Rotterdam and Breda in the Netherlands. Breda is worth a visit too btw. So for visiting the other places you need seperate tickets.

Travel planners: https://www.ns.nl/en/travel-information (national) and https://www.nsinternational.com/en for international planning. Or this Belgian website: https://www.belgiantrain.be/en (there is a link to international travel on the same page)

Nigel has given an excellent explanation about the 2 different types of trains. I just wanted to correct 2 small errors in this paragraph.
“ The International train also uses the "high speed" line between Amsterdam Centraal and Rotterdam / Roosendaal but more slowly, and does not use the very high speed line in Belgium.”

The high speed line runs from Rotterdam towards Breda, not Roosendaal. The high speed line crosses the border just south of Breda, at Hazeldonk. The first train station on the Belgian side of the border is Noorderkempen. Both the Thalys train and the regular international train use the high speed line all the way up to Antwerp. The only difference is that the regular international train stops at the intermediate train stations Breda and Noorderkempen, whereas the Thalys doesn’t.
I live in Breda and I’m a frequent user of the international train in both directions, either to go to Amsterdam or Schiphol or to go to Antwerp. The Thalys doesn’t stop in Breda, but we still benefit from the highspeed line running right by us. I can take the train to Antwerp, exit the train, remain at the exact same platform, wait 15 minutes, board the Thalys and train to Paris, all in less than 3 hours.

Another thing to add. If you buy your tickets for the regular international train more than 10 days in advance, you can benefit from the early bird fare. Tickets for the regular international train aren’t tied to a specific train on a specific time. You buy a ticket for a specific day, and you can take any train from Amsterdam to Brussels on that particular day. You can even interrupt your journey, in Antwerp for instance to look at the stunning train station. Or in Breda of course :-)

Thalys is a fast(er) train, but all trains in the Netherlands are
limited to 160km/h (and only 130 in most places) because of the sandy
substrate.

Are you sure about that? There is a high speed line from Amsterdam via Rotterdam to Antwerp and last time I checked Thalys were able to run at 300 km/h on that line.

The Thalys runs at 300 km/h in the Netherlands, except on the stretch between Schiphol Airport and Amsterdam. The international Intercity runs on the high speed line, but its maximum speed is limited at 160 km/h.