Hello,
We will be catching an overnight international flight from the US to Amsterdam Schiphol arriving at about 10:30 am in early July and then want to take the train to Brussels after that. I know there is the IC train and the Thayls train. We don't mind paying extra for the Thayls train, but don't want to lose our money if our plane is late. Will we be able to buy tickets for the Thalys train upon arrival? Will the IC train be available if the Thalys train is not available?
Thank you.

IC trains don't even take reservations - so can always board - 1st class more sure to have empty seats. Thalys may be full and full-fare is so expensive.

For lots on trains check www.seat61.com; BETS-European Rail Experts and www.ricksteves.com.

It's possible the Thalys will be full, but what is certain is that you will pay a king's ransom for the fare if you buy it on the spot. It's a journey of under 2 hours, so the smartest thing to do, IMO, would be to allow yourself PLENTY of time after landing to catch the train. It's not exactly a major hardship to sit in Schipol for an hour or two and have a coffee/snack, even if after an all-night flight.

Thalys discounted tickets according to menachem are much harder to get than before - just hop the IC train that takes an hour longer but saves so much money but check www.thalys.com to see what is available.

IC trains would be my favorite.
You are not bound by any reservations. No matter if your plane arrives an hour early or two hours late, you have one train per hour.
So, in a worst case scenario, you'll have to wait 59 minutes at Schipol airport.
In case your hotel is near the historic city center of Brussels, IC trains will stop at Brussels Central - which may be much more convenient compared to Thalys which makes you go to Brussels Midi, and then go back to Brussels Central with another train.

cowboy spots key item -ICs stop at Gare Centraal - right near the heart of town - Zuid or Midi station is a few kms west and yes you have to take a commuter train back to Centraal - not many hotels around Zuid-Midi - not that nice an area IME though not unsafe - just a busy busy area.

Thank you so much. This is very helpful!

IC trains may be full, but they won't be sold out. The Amsterdam - Schiphol leg is the most busy, then Schiphol - Rotterdam moderately so, and Rotterdam - Antwerp not busy at all. Antwerp - Brussels may be a bit busier, because it's also used by commuters (same as between Amsterdam - Schiphol and Rotterdam) You'll have seats. Especially if you get on at Amsterdam Central: usually the Brussels IC will be standing there already, about 10 minutes before departure, because it's the terminus station for that line. So if you arrive a bit early, and walk away from the stairs towards the end of the platform (at the most western access tunnel) you can choose where you want to sit.

Thank you so much! We'll be going from Schiphol to Brussels (looks like a stop in Rotterdam at the time we were thinking of) and then Ghent to Amsterdam Central (with a couple more stops on that leg). What are the normal rush hour times? I was guessing like 8-10am and 4-7pm?

7 - 9 am and 4.30 - 6.30am: those are the official peak hours, coinciding with the morning and evening rush hour. In off peak hours, trains are much quieter, because there will be fewer commuters.