We got the following information via email:
Arriva is running the 3-country-train (Aachen (D), Heerlen (NL), Maastricht (NL), Liège-Guillemins (B).
It is of course sad to see the old Belgium trains disappearing on the Maastricht-Liège route but the good news is that (only) between Liège and Maastricht the train is, even it is an Arriva train, operated on behalf of NMBS (Belgium part) and NS (Dutch part as far of Maastricht) and tickets stay valid for this train (note: the train is running further to or from Aachen via Heerlen and Valkenburg to Maastricht, for this part the NS/NMBS and so FIP ticket is NOT valid, but it's not 100% clear).
What is unclear for me:
Do you need a NS coupon for traveling between Liège-Guillemins and Maastricht
What is clear for me:
NMBS/NS tickets remain valid between Liège-Guillemins and Maastricht
On this site/page: https://www.arriva.nl/over-je-reis/met-de-trein/de-drielandentrein/ you can find the information and also on this website https://www.b-europe.com/NL/Bestemmingen/Maastricht
ARRIVA
Existing tickets remain valid
In all three countries, existing tickets issued by NMBS, NS and Arriva remain valid.
You can travel between Liège-Guillemins and Visé on a Belgian ticket.
also untill the border
You can buy a ticket via this website for the journey between Liège-Guillemins and Maastricht.
Between Eijsden and Aachen Hbf, you must check in with Arriva, and Limburg products and fares apply.
Between Eijsden and Maastricht, you can also check in with NS.
also from the border
Between Heerlen and Aachen Hbf, you can also travel using the Übergangstarif.
Are you travelling between Herzogenrath and Aachen Hbf? In that case, the German Rheinlandtarif applies
NMBS
Since June 2024, NMBS has been operating the Liège–Maastricht route in partnership with Arriva and NS.
SNCB still operates the train service between Liège-Guillemins and Maastricht, with stops at Bressoux, Visé, Eijsden and Maastricht Randwijck. However, this service is now provided by Arriva’s blue-and-white trains, which previously operated in Germany and the Dutch province of Limburg. Beyond Maastricht, the train continues on to Valkenburg, Heerlen, Landgraaf, Herzogenrath and Aachen, earning it the nickname the ‘Three-Country Train’.
SNCB tickets and season tickets are valid between Belgium and Maastricht.
It's hard to find written FIP approved information, but I've uses my FIP a few times without any concerns. I've asked on my office and the product manager for this train and I've received a positive answer for using FIP (it's the same condition as NMBS/NS ticket)
We got the following information via email: