Check with the national railways, DB, SBB are the best.

You should generally only trust the operators' own timetables. The rail planner app in particular cannot be trusted.

I don't see this option anywhere. Could you please give details of this journey?

Indeed always use the national railways websites or apps. It is the most reliable place for sure.

Trainline and Happyrail don't show your connection because it takes a bit longer than going to Paris all the way with high-speed trains.

I'd recommend the 07:15 - 18:10 route as it only has optional reservations (4,50€ all the way) while you'd pay 70€ in mandatory reservations via Paris (which could be sold out) + RER between Paris stations

Or are you looking to buy normal tickets ?

@rvdborgt I can see it, at least on the SBB App. Changes in Biel/Bienne, Basel SBB, Frankfurt (Main) Hbf and Utrecht Centraal

Right, this is also how I would travel. But then, I know to force "via KÖLN” to get the desired results. Not many ordinary people know this and not all planners allow this so I don't assume too much.

Yep true, I guess they searched for reservation-free itineraries on interrail.eu. I don't really know how it would show otherwise.

I'm doing this route soon but in the reverse direction from Utrecht to Switzerland and I plan to change in Köln as it could be tight in Frankfurt with the usual delays...

Thank you all for your quick responses!

But if I know which train I want to take from the specific national railway operator but it does not show up in the interrail planner then how do I book a seat when reservation is required?

On your route reservations are all optional.

The whole journey Basel - Frankfurt - Utrecht costs 4,50€ through bahn.de or DB App.

Usually you can buy them on the companies websites but there are exceptions as usual…

See this thread for advice :

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

or this very helpful website

https://www.seat61.com/interrail-and-eurail-reservations.htm