For your hypothetical journeys you won't need a supplement. 

But that's just true because this only exists on the three DB connections mentioned:

• Cross border services Germany / France with ICE / TGV. 

• DB-ÖBB-EC to Italy via Brenner.

• ECE to Italy.

So it doesn't exist for ICE Germany / Belgium or regional trains. 

No supplement needed, reservation is optional, extra ICE ticket is needed until Aachen Süd(Gr), which is available for the standard fare with any BahnCard discount (25 or 50% discount). Alternatively, you can buy a (Super)Sparpreis to Liège (with any BahnCard discount). That can be cheaper than the normal fare to the tariff border point. If you have a BahnCard 100 (DB Network season ticket), then you don't need any extra ticket, since a BahnCard 100 is valid until the tariff border point, so together with your Interrail, the whole route is covered.

That's the same as above, just without the BahnCard discount.

You will also only need a ticket to Aachen Süd(Gr), which from Aachen Hbf (if you're starting there) should only be about €3 for the regional train (BahnCard discount possible). Or no extra ticket with a BahnCard 100. The first station in Belgium is Hergenrath, by the way.

Some railways are now selling tickets from/to the border tariff point as tickets from/to a real station, with 100% discount in the country for which you have a pass. The price should be the same as a ticket from/to the border tariff point. Exceptions exist for international services with mandatory reservations, such as the ICE/TGV services between Germany and France.

Achtung!!!! der ICE9553 fährt nur bis Mannheim bitte plane deine Reise über Bahn.de oder den DB Navigator 

Das Problem ist leider das deine Grenzüberschreitenden Züge nicht mehr in Forbach halten somit gibt es hier keine “leichte Lösung” und du gehst am besten mal in ein DB Reisezentrum oder eine Erfahrene Bahnagentur wie Kopfbahnhof (Yorckstraße), Bahnagentur Schöneberg (nähe Julius-Leber Brücke) oder die Bahnfüchse in Berlin-Köpenik

Ich persönlich hätte meine Inlandsreisetage für die Fahrt nach Bordeaux verwendet und für meine Fahrt nach Zermatt denn einen DB Sparpreis bis Basel (es würde auch ein Flixtrain nach Basel fahren) 
@SaraR  
Ich hätte noch eine etwas leichtere Verbindung als Vorschlag 
 

@seewulf

14.02. Bordeaux → Berlin

09:46 TGV 8530 → Paris

13:10 ICE 9553 Paris EST → Frankfurt

17:14 Frankfurt → Berlin

 

 

Achtung!!!! der ICE9553 fährt nur bis Mannheim bitte plane deine Reise über Bahn.de oder den DB Navigator 

Das Problem ist leider das deine Grenzüberschreitenden Züge nicht mehr in Forbach halten somit gibt es hier keine “leichte Lösung” und du gehst am besten mal in ein DB Reisezentrum oder eine Erfahrene Bahnagentur wie Kopfbahnhof (Yorckstraße), Bahnagentur Schöneberg (nähe Julius-Leber Brücke) oder die Bahnfüchse in Berlin-Köpenik

Ich persönlich hätte meine Inlandsreisetage für die Fahrt nach Bordeaux verwendet und für meine Fahrt nach Zermatt denn einen DB Sparpreis bis Basel (es würde auch ein Flixtrain nach Basel fahren) 
@SaraR  
Ich hätte noch eine etwas leichtere Verbindung als Vorschlag 
 

 

@SaraR Nur falls du das in Erwägung ziehen solltest: Die hier gezeigte leichtere Verbindung ist keine praktikable Idee; der TGV ist bereits ausgebucht.

Die Fahrttage in Deutschland zu verwenden und für die Verbindung in die Schweiz die Fahrkarte bis Basel zu bezahlen würde für sinnvolle Verbindungen auch erheblich teurer werden als das Geplante.

So to put my mind at ease:

The original post had this as the translation.

There is a "pass surcharge 2" tariff for connections to France, which you get if you have a network card that is only valid in France. It covers the German part of the route.

You need the "Passport surcharge 1" tariff, which is not exactly cheap, for the cross-border ICE / TGV anyway, even if you use the travel days in your home country. But with "passport surcharge 2" you no longer need it.

But the only thing that helps is going to the travel agency at the train station. I would have the price for the pass surcharge 2 checked there for the specific date. 

You can really save (on the route) but probably only by a cheap fare to Saarbrücken and then with the slow train across the border to Forbach or Metz. Delays in time of course. 

Incidentally, the official limit on the route is actually on the open route and not in the station. Border tariff point is Forbach (fr). In this case, however, this is entirely theoretical. Buying a ticket to Forbach (i.e. the first station after the border) is unnecessary and would be too expensive anyway (in the ICE / TGV).

You may now understand my confusion (still not fully clear) as the inference of the post is that you have to pay for all journeys across the border, but if on the ICE/TGV it is extortionately expensive even if you have a fully valid pass for both countries. 

So if I want to travel on one of these ICE/TGVs with a full Global pass how much extra do I have to pay and where is it paid?

Ok, I think I got it (what the misunderstanding has been) - at least I hope so.

The missing part is: You don’t need a supplement across the border in general (we’re talking about the german-french border only), you just need a supplement across the border for the german-french corporation ICE / TGV.

Next thing: It’s not cheap, but it’s not that expensive compared to a normal TGV reservation. The “Passzuschlag 1” is 16 € if bought at a german station, the reservation for an ICE / TGV in France is 10 € or 20 € - so the 16 € supplement may even be a better deal sometimes. 

And: If you’d buy a ticket to Forbach and would use a railpass from Forbach to Paris, it would be really expensive if you would like to stay in the ongoing train. 

So just to close this down - If I want to cross the border on a TGV/ICE with a global pass and a mandatory reservation I would still have to pay an extra supplement of 16 euro? 
 

You need to pay 16€ wich is Reservation + surcharge. You can’t book only the reservation or only the surcharge. 

No, the supplement does include the mandatory reservation. In fact, if you’d ask for a mandatory reservation, they will sell you this supplement. Maybe we could say it’s just the name... 

But many of the posts suggested that wasn’t the case - you had to buy the supplement separately on any train over the border. I asked earlier if it was incorporated in the reservation for pass holders - that would have solved the issue instantly. 

So the advice to anybody without a in/out day is still get to the tariff point with your least cost domestic fare for the appropriate train, and then activate your pass for the first train at or beyond the tariff point.

I assume also that anybody travelling on this ICE/TGV service is also totally unaware that part of their ticket price includes a fee for crossing the border.

I admit I still don’t understand it fully but enough is enough.

I think you tried to touch on a bit of French-German "Bureaucrazy" discussion here @Yorkie . This didn't really classify for light Sunday evening reading... The general advice is to steer way clear of it when it happens ;) 

But at least I understand the terms now. Some other railwaybmystery solved. Thanks people!