We are arriving in Brussels Airport via train scheduled to arrive at 10am and have a flight to the States at 1120, we are US citizens. Will 1 hour be enough time to make our flight?
With the minimal information you've given, the only sane answer is that "no, it will not be enough time".
Flights to the US have a 60 minute cut-off for check-in and baggage drop. This is strictly enforced (at least for check-in, and almost always for baggage) as it is based around US legal requirements.
This will give you 20 minutes to go from the train arriving (if it's on time!) to having completed the checked-in process. Whilst this might be possible, the risk is simply too high to consider it even close to safe.
If you have no bags, and if you can check-in online (which is NOT guaranteed, especially with the new US rules around extra security that require many passengers to check-in at the airport), and if you are not selected for additional security (those same new US rules), then yes, this is possibly enough time - but even I would never risk it, and I common cut things tight...