Amsterdam to US connecting in London

Simply checkin online 24 hours the night before print the boarding pass or download on mobile device ; be at the airport two hours before the flight or the time recommended by the airline you bought the ticket from. If you are just using carry on you can go to gate as long as you have the boarding pass. If you are checking a bag you use the bag drop of the airline and go to the gate.

When taking the train there is a passport border control check in Brussels.

When flying from Amsterdam to London you will go through passport control on the way to the gate and the border control officer behind a glass will stamp your passport

First the trains. When you check in for the Eurostar in London you will pass through 2 passport checks. One to exit the UK, then one to enter France. Once past those, you're done with border controls until you fly out of Amsterdam. There are no border checks (normally) within the Schengen zone. Border control in Brussels is usually for people heading TO London, not from it. If I understand your post correctly, you will be taking the train from Paris, so no border control necessary.

Can't answer your question about check in times for A'dam, but you absolutely need to be at the London airport at least 3 hours early.

As written these are not connecting flights but separate.

So the advice would be to arrive at AMS at least 2 hrs before your flight to LHR as it’s a short haul. Arrive at LHR 3 hours before your flight as its long haul. If you have checked luggage you need to pass through the UK border to retrieve the bags and re-check.

Of course if you amended your return ticket to the US as a true connecting flight then your bags would be checked from AMS all the way through to the US and you do not pass the UK border. Quicker, simpler and your connection is protected.

Yes, sorry, my headline was unintentionally misleading. We had originally planned to stay in London a few more days on the way back. I’m going to see if Delta will let me change our flight back to the US straight from Amsterdam. That would certainly be the simplest and most logical solution. Thanks everyone for the input!

I’m going to see if Delta will let me change our flight back to the US straight from Amsterdam.

Oh boy if you bought the cheapest ticket initially they probably charge a change fee

Yes, sorry, my headline was unintentionally misleading. We had
originally planned to stay in London a few more days on the way back.
I’m going to see if Delta will let me change our flight back to the US
straight from Amsterdam. That would certainly be the simplest and most
logical solution. Thanks everyone for the input!

Of course, Delta will let you change your itinerary. They'll make money off the change fee.

As for flights, you can check in online up to 24 hours in advance so do that. Download the airline app to make it that much easier for you.

Make sure you arrive at the airport 3 hours in advance of your transatlantic flight.

I had never thought about customs checks on international train
travel.

Not sure why you would even give customs ANY THOUGHT unless you have something to declare.

The trick of arriving at LHR 3 hours before your trans-Atlantic flight departs is that it will take you an hour or hour-and-a half to get off of your incoming flight and to the check-in point for that LHR - USA flight. So a three-hour "gap" between flights isn't enough. You really need more like 5 (especially to provide a cushion in case the AMS-LHR flight is late).

If you plan this type of travel again, purchase a ticket that allows you to fly into one city and back home from another. Air France flies from Amsterdam also.