It’s very difficult to help if we don’t have details.

Please provide dates, travel hours and times, and if possible a screenshot.

In the german version of the app all connections are shown as "reservation mandatory".

And indeed this bus does have a mandatory reservation (it's just a minibus). It needs to be done at least two and a half hour before departure.

I don't know if there's a real reduction for passholder. At other CFL services you do in fact pay the normal price but I don't know for sure for this one (because it's the most expensive one). You'd need to ask at the station. 

The normal price (that does include the mandatory reservation) is 21 € and can be done online: https://www.cfl.lu/fr-fr/ticket/internationaldetail/gare-lorraine

Thanks Hektor

The ‘english language’ version of the app also now says ‘seat reservation required’ with N/A shown as the reservation cost.

Maybe the easiest thing to do is to try & reserve seats using the app which I will do as soon as I purchase the Eurail pass.

Thank you for your response, it is appreciated.

The app cannot book anything, it only redirects you to other websites such as eurail.com and others.

I’m 99% sure that the “reservation fare” is the same one as the ticket fare (21€). This is the same on the Luxembourg - Saarbrücken service as Hektor said.

Thanks thibcabe

I do understand that the app only redirects. What I should have said is that I will use the link Hektor provided.

Mea culpa    

I have the same problem but from Luxembourg to Lorraine. At the ticketoffice in Luxembourg they said that there interrail is not valid there. But mij ticket shows otherwise.

Maybe the reservation fee is the same as a ticket price.

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Yes that's the thing. Interrail is "valid" but the reservation costs the same as a ticket.

Similar on the Luxembourg - Saarbrücken coach service.

Thanks for the feedback anyway. :)

I removed the bus leg part of interrail day, as I have a valid ticket and I see no reason why the bus compagny should get any money from interrail/eurail. Smal revenge.