Train from Amsterdam to Berlin

Others will know more, but on many inter-country trains you can reserve seats
for a small fee, even in 2nd class.

But definitely check 1st class, sometimes the price difference isn't all that high.

If the train originates in Amsterdam, then if you board early getting 4 seats
together even in 2nd class should be doable.

You should be fine finding four seats together in 2nd cl. as it likely won’t fill up in Amsterdam, but in the cities it stops at after Amsterdam. If you’re really worried about this, just board the train a comfortable 30 minutes before it departs because usually most passengers won’t board until the final 15 minutes.

First, you should fly. You are on vacation, time is precious. Think about what 5 more hours in Berlin are worth. Also realize that you will have to bring all your own food and drinks with you, as the direct train won't stop long enough for you to get off an find provisions.

If you take the train the 1st class compartment is a small section walled off with glass from the main (2nd class) section. It's on one end, directly over the trucks. It can be quieter, but not much. The seats are slightly better. usually there are 8-12 seats in 1st class. I travel by train often and have never seen the need for 1st class, but I try to fly or drive if it's more than a 4 hour train ride. It can actually be cheaper to fly.

First, you should fly. You are on vacation, time is precious. Think
about what 5 more hours in Berlin are worth.

This is incorrect. Flying will not give you 5 hours more. It may give you 2 hours more. But don't forget that flying involves a lot more "friction" at both ends.

Also realize that you will have to bring all your own food and drinks
with you, as the direct train won't stop long enough for you to get
off an find provisions.

Why is that a big issue? It is not as if the food and drinks on planes are that great nowadays. Get yourself a picnic before you board the train, and just enjoy the ride.

If you take the train the 1st class compartment is a small section
walled off with glass from the main (2nd class) section. It's on one
end, directly over the trucks.

This is incorrect. On the Amsterdam - Berlin train the 1st class is tow carriages, separate from the rest of the train. That is common on long distance trains in Germany.

It can be quieter, but not much. The
seats are slightly better. usually there are 8-12 seats in 1st class.
I travel by train often and have never seen the need for 1st class,
but I try to fly or drive if it's more than a 4 hour train ride. It
can actually be cheaper to fly.

So you actually do not travel by train a lot, and seem not to know what long distance trains in Germany are like. I travel a lot in Germany, even on longer distances, and I travel by train, because my time is precious...

Currently the Amsterdam - Berlin trains are operated with older (but still perfectly servicable) rolling stock. 1st class is 6 seat compartments. Second class are open plan coaches.
When you buy tickets in advance 1st class is often only a little bit more than 2nd class, and in that case I would go for it. But I would also get a seat reservation. In summer these are included, but the rest of the year you have to get those separately. Personally I think the German railways should always include a reservation with a long distance train ticket, but that would unfortunately not go down well with the Germans for some reason...

just board the train a comfortable 30 minutes before it departs

If you go to the station 30 minutes before departure the train won't even be there yet. I will probably pull up about 5-6 minutes before departure. Just get seat reservations and you do not need to worry about seats.

Regarding the comparison of planes versus train. Don't forget that the train ticket is all you will be paying. No extra charges for luggage. No extra travel to/from airport. A month from now I see first class for 87,90. Easyjet may appear cheaper, but their headline fare does not include luggage.

I don't think you should fly, total travel time is roughly the same. And when travelling by train you can actually use the time as you want, when flying there is a lot of time wasted queueing and waiting.

Yes you can reserve seats in second class, reserve four seats around a table and you will get a nice trip.

Also if you travel 1st class you can use the 1st class lounge at Amsterdam Central on Platform 2 before travel. The Bistro car has been withdrawn from the trains but there is a trolley service according to the Man in Seat 61.

There were brand new ICE-L trains due in service by now (which will reduce the journey time by about 30 minutes), but those have been delayed until, apparently, 2025.

All you do on the train is board and travel city centre to city centre and see the countryside. And you can get up and stretch and wander around at will- not jammed in your seat as by air.

By air you have the travel (and extra cost) of travel to and from the airport at either end, you see nothing, and have all the security stuff and being at the airport well in advance to cope with. It is very unlikely that, taking all the costs into account, the air fare will be cheaper.

Travelling by air is also far more environmentally damaging.

At Amsterdam Central (and most large European stations) few if any trains are platformed 30 minutes before departure- not even the night trains. Most people board within the last 15 minutes because that is the length of time the train is on the platform for.

...we are choosing the train over taking a flight because we want to
see the countryside.

The first part through the Netherlands is not spectacular, but pretty nice. You'll probably be disappointed with the scenery on this route through most of Germany. Also, you'll be moving quite fast, which turns much of it into a blur. I'd bring a good book.

Meals on long trips: I actually prefer to make long trips maybe 60-90 minutes longer so I can stretch my legs and find whatever kind of food I'm in the mood for at/near the station in question. The DB site permits you to schedule a stopover in the city of your choosing for various lengths of time.

The questions you ask - and the questions you did not - are addressed professionally by Mark Smith, the "Man in Seat 61." His website and tips are highly regarded by nearly everyone. Below is a link for his advice on your route:

Train from Amsterdam to Berlin

A number of the forum's posters have done that trip and described it here. It would be wise to do a search and read about their widely varying experiences. I wish I had read online reviews of the experiences people were having on that journey because we certainly didn't receive what was described. It's not something my partner will allow me to arrange again.

The flight from Amsterdam is one hour, 25 minutes. it's an internal flight within the EU, "Friction" is minimal. The train is 6 hours. that's if (a really big if, lately) it's on time. Thanks, anyway, I'd fly.

KGC, you do you, of course. But disregarding the large carbon footprint of flying, you are also disregarding the time required to get to and from each airport., and the 2 hours needed to be at the airport before the flight. So at MINIMUM that 1 1/2 hr flight will take 4 1/2 to 5 hours before arriving in central Berlin. Assuming they check in online and only have carry on luggage. Thats IF there are no delays in the flight departure. With all of the attendant hassles that airports have. OP can choose which ever mode of travel they prefer. But they should know the time factors that go into flying.

but there is a trolley service according to the Man in Seat 61.

I wouldn't want to rely on that, but rather buy my food in advance. If the train is quite full, the trolley either won't come at all or it could be sold out by the time it arrives at your seat. It has also happened to me that it was there but hardly had anything on offer: “I didn't get coffee in Munich, it's only available from Würzburg” the guy said to me recently and kept me waiting for two hours.

To that 1h25 you have to add the time you need to be at the airport (check in at Easyjet closes 40 minutes before departure), the time to travel to Schiphol, and the time from BER to down town Berlin. The time to wait for your luggage. And a sufficient buffer. Downtown to downtown is never less than 4 hours when flying within Europe.

Plus the time lost due to the schedule not being optimal. The train goes every two hours, so it is easier to pick a departure that fits your overall plan for the day. I prefer to leave after a leisurely breakfast, and arrive before dinner. Works perfect in this case with the train.

We took the direct train from Amsterdam to Berlin last month in June. It was an easy trip. We booked 2nd class and paid a couple of bucks for a seat reservation (we don’t like surprises). We booked a couple months out and there was plenty of availability for seats. I will say the train wasn’t as inexpensive as I expected, but it was still cheaper than the flights we had researched. It was not a scramble when boarding in Amsterdam, but we took an early 8am train. The seats in 2nd class were fine with plenty of legroom (way more than a plane). We travel with carryon size bags only, so no concerns with luggage space either. We got a to-go breakfast from our hotel so we weren’t concerned about the trains foodservice, so can’t comment on that. And as others have stated, there was no lengthy trip from city center to the airport, and no security lines or checks. I think for a train journey that will take 5 hrs or less, I would choose the train over flying but that’s just me. A lot less stressful overall, IMO.
You do have to make sure you actually get on the correct train, so just read the platform numbers and check the name and number of the trains destination when you board. That’s always my secret fear:)

I'm not sure what the discussion is about "wanting" seat reservations. It's my understanding that seat reservations are required on this route; the NS* app says they are. And FWIW, we were unable to get seats this week (1st class) on any train except the 3:30 pm train getting us into Berlin Hbf at 10 pm. So plan ahead.

*NS = Dutch rail service

The OP hasn't said when they are travelling, but according to the Man in Seat 61 reservations are required only from 1 June to 1 September, a new thing for this summer.
That is confirmed by the DB website.
The full walk up full fare is 142.80 euro, + a further 106 for 1st class. But looking today for mid September train specific Super Sparpreis fares are as low as 38 Euro in 2nd class, or 74 in 1st class.