3 Country Connection time Brussels Midi / Zuid

1. None whatsoever.
2. About 10 minutes should be sufficient.
3. What checks? You go to the station. Find the train. Get on. Then on the train you show your ticket. Repeat in Brussel. If a delay makes you miss your connection in Brussel go to the Eurostar ticket office and have your ticket made valid for the next train.

Has anybody noticed any increased time in completing checks on Americans since we've become more belligerent to our traditional friends

Regarding Immigration and Customs in the countries within the Schengen zone, you go through Immigration (Passport Control) on your initial entry into any of the Schengen countries. You don't encounter another Passport Control until you leave the Schengen zone. In your case, since you will already be in France, you won't have any requirement for additional Immigration checks because Belgium and the Netherlands are also in the Schengen zone. Occasionally there may be police who board a train at a border crossing and ask to see everyone's passport. But 99.9% of the time it's just a glance and they move on.

That said, who knows if barriers to entry will or will not be put in place in reaction to any future aggression.

Trains are a bit different to planes in terms of connection time, with the exception of the Eurostar between Europe and London. For that train you do your immigration formalities before you board the train, as there is no formal immigration process on arrival at the destinations. You need this extra step because the UK is not within Schengen. But for trains within Schengen, as CJean explains, the "connection time" really just consists of finding the right platform (track), which takes minutes. With some connections you need only cross the platform to the other side. You can look up your trains on https://www.b-europe.com/EN or https://www.sncf-connect.com/en-en/ or https://www.nsinternational.com/en , putting the entire journey on there and looking up how much time you need (or have) for the connection. The information displayed there will tell you what train company the connection is with. But mostly even with a short connection time you will make your train.

I am not American myself but I don't think Europeans will foist their anger on you personally because of the political situation. But current circumstances make for some interesting political discussions over a beer in the pub one night. Look at this as an opportunity to exchange points of view, not hostilities :)

Lavandula

Prior to the Schengen Treaty you did not have to go through immigration before boarding an international train either. That has always been a Channel tunnel trains peculiarity, and it has hampered Eurostar's expansion.

The normal way was always to do immigration and customs on the train while it was crossing from one country in the next.