We flew into Brussels on a Tuesday morning. News stories and at least one trip report had warned of multi-hour waits for entry. There were 8 EES kiosks for non-EU passengers off to the side and many people were just directed past them to wait for an officer. We were able to use the kiosks (and I should have coached one of the kids ahead of time that we did not have a visa and that end date meant Schengen not just Belgium). As others have reported, at the end of the process, there is a brief message about whether to see an officer. It seemed like almost everyone who used the kiosks was asked to see an officer but at least it was a shorter line. From landing to buying train tickets was about an hour.
I bought train tickets easily on the SNCB app, but couldn't get the QR code to expand and scan at the gate. The employees helped us through the gates at the station. Later, on the train, the conductor checking tickets told me to close all my apps and then reopen only the SNCB app and the QR codes expanded to be scanned. I wanted to use the app because it allowed discounts for the kids and for off peak travel as a group.
On the two level train, the bottom level had folding seats along the side and plenty of space in the middle. It seemed designed to cram in commuters. I'm glad we checked the top level because it had two seats on each side allowing facing forward and a small luggage area. On later trains, the bottom level also had seats.