Checked Baggage and Customs - Getting conflicting information and looking for clarity

You will go through passport immigration control and security screening on the way to the gate in London. Customs is the area where you pick your bag up and walk through to landside. The British understand it this way. Since I am assuming that you bought your flights all on one ticket you will only go through customs at your final destination which is Rome. The bag tag and claim check for you bag if you are checking one will have this listed on it. Either way when you check in in Seattle the passenger agent will clarify this by telling "your bag will go all the way to Rome." I use to work for an international airline and that was the procedure.

There is no passport immigration check on train travel between the countries you are traveling through.
Remember what many here in America refer to as customs means passport control in Europe.

If it is one ticket, your bags will be checked through to Rome. Nigel just posted a link to the Gatwick airport transit procedures. If you do not have to change terminals at Gatwick, you don't have to go through UK passport control. Here it is.
https://community.ricksteves.com/travel-forum/transportation/airport-security-information

There are no customs between Italy and Germany. Italy, Germany, and Austria are all members of the EEC. No customs checks, They are also members of Schengen Treaty zone, no passport checks between them. However, you will give your passport to the attendant on the Nightjet, just in case. They give it back when they wake you for your final stop.

I'm assuming you are flying Alaska Air from Seattle to Los Vegas, then British Airlines from Los Vegas to London and Rome. If so, I would tend to listen to British Airways. However, when we flew from ATL to Basel, Switzerland via Amsterdam, we went through customs at Amsterdam.

Since Italy, Austria and Germany are all part of the EU, you won't be awoken at the border. You'll go through customs when you board. It does remind my of my first trip to Europe back in 1966. One leg of the trip had us travel from Lucerne, Switzerland to Venice. We did stop at the Italian border and officials came through the train and checked our passports.

I don't envy your flight with two layovers. That's going to be a killer trip. How many hours between leaving Seattle and arriving in Rome?

However, when we flew from ATL to Basel, Switzerland via Amsterdam, we went through customs at Amsterdam.

I think not. I think you went through Schengen Immigration at Amsterdam. You went through customs in Basel, if you had your bags checked through from Atlanta to Basel. The thing is, customs in Europe is such a nothing that people often don't realize it. You just walk out the "Nothing to declare" door.

Immigration is passport control where you are official allowed into Schengen, after checking that you are OK.

Customs is taxes paid on imported goods. Since you are presumably not bringing any goods for sale, just personal belongings, there is nothing to declare, so you simply walk out the door.

You are getting conflicting answers because you are confused.

Customs and passport control are two different things. You will only go through customs in Rome. You will go through customs after you collect your bag.

Passport control is totally different and has nothing to do with your bags. Americans tend to lump Passport control and customs into one thing and call it customs, but that is incorrect. The agents you spoke to were thrown off by the terms you used. You’ll go through passport control at Gatwick (unless you stay airside) and in Rome (point of entry in the Schengen).

Once you are in Europe, there are no borders as all countries you will visit are part of the Schengen Agreement. You will not be disturbed during your train travel. It’s like crossing a state line in the US.

You will definitely go through immigration (for people) and customs (for things) in Rome because you are entering the Schengen.

Whether you go through immigration and customs at Gatwick depends if you're entering the UK or staying airside to transfer flights. If this journey is all one ticket and your bags are checked through to Rome then no, you will not go through immigration and customs at Gatwick.

And please remember what "customs" entails--walking through the exit doors in the FCO arrivals building, using the lane that says nothing to declare. No questions, no baggage examination. It is not like arriving in the U.S.

Although there shouldn't be Immigration agents coming into your NightJet compartment, I trust you realize that the train makes multiple stops through the night. Even as a youngster with no sleep issues, I did not sleep through the braking, station noise and later acceleration accompanying each stop.

These are the post-midnight stops for the train departing at 9:55 PM

Tarvisio Boscoverde dep 03:05 AM
Villach Hbf dep 04:00 AM
Schwarzach-St.Veit dep 05:20 AM
Salzburg Hbf dep 06:27 AM
Rosenheim dep 07:36 AM
München Hbf arr 08:19 AM

You have a substantial chunk of time before 3:05; with luck you'll be able to sleep pretty well during that period. I had no such luck on my last night-train trip--and I do mean "last" in both senses of the word. But that was a trek through southern Italy, and I think the rough condition of the rails was a significant factor there. Still, I recommend assuming that you are unlikely to be at your mentally sharpest that first day in Munich.

Yup, night trains are the absolute worst.

Thank you everyone for all of the information. It is so helpful!

Wow, I'm confused with this thread.

If you are just changing planes at Gatwick, and if you have checked baggage that can be checked through (you are traveling on one ticket), you will just use Flight Connections at Gatwick to get to your Rome flight. No passport control, no customs. (When you check in in Seattle make sure they tag your bags for Rome.)

When you arrive in Rome, you will go through Passport Control and Customs. Passport Control here covers the Schengen area that includes Italy, Germany and Austria among others. No passport control between these countries and you won't be woken. Or at least your shouldn't be.

Ugh...night trains. I have a DB horror story for the overnight from Paris to Munich. I don't think that DB runs the night train with sleepers any longer. If they do - Top Tip: Request/reserve a handicap berth.