You can take ICE trains from Cologne Messe/Deuz (instead of Hauptbahnhof) to Frankfurt Main Station, they run via high speed line and you can catch the Nightjet/Intercity train to Zürich.

Check the DB Navigator App or bahn.com, this is the only uptodate for germany. On ICE reservation is only optional. Same for the seating carriages of the Nightjets Germany-Zürich, they run as IC. 

When you don't get the Nightjet, there are many trains to Zürich from Frankfurt. If you need a new travel day than ask via customer service to get one more via this form: https://eurail.zendesk.com/hc/en-001/requests/new

The S-Bahn local communter trains run via Collogne Main Station with delays. All other trains are rerouted.

It seems this is doable if I go via Dusseldorf and then join the IC/nightjet from Mannheim according to the DB site. But rather than a sleeper I’ll be in a regular carriage - I’m assuming there would be nowhere but the usual overhead racks/luggage compartments at the end, I wouldn’t be comfortable leaving my valuables there while asleep.

I can’t leave Brussels until 7pm as I have an important meeting until then

If I were to say travel to Dusseldorf this evening, then do the rest of the journey the next morning would Interrail give me an additional travel day due to the disruption?

I have a Thalys reservation from Brussels to Cologne, what’s the best way to change that to Brussels - Dusseldorf? A new reservation would be necessary whether I go overnight or split the journey.

Am I also likely to get a refund from ÖBB for the altered nightjet service that’s now no longer of use to me?

If you are picking up your nightjet in Frankfurt or a later stop then your sleeping accommodation should still be available so no need to spend the night on seats.

The current advice given on OBB app is to only use ICE 251 from Amsterdam at 18.38, due Koln HBF 22.05, Frankfurt Main HBF 23.24.

They don’t give a departure time for NJ403 for Frankfurt.

ICE 251 is currently listed as late on DB app, (Koln at 23.33) this may help you connect with it.

DB app gives arrival at Koln for 22.00 from your Thalys 9471 changing in Aachen and Koln-Ehrenfeld

If it is not possible to get to Koln Hbf by train then a taxi from Ehrenfeld should take less than 20 minutes.

Intercity IC60403 is the Nightjet NJ 403 Amsterdam-Zürich. The train has 2 different names. Sleeper and Couchette run as Nightjet, seat carriages as Intercity.

Just a few things to clear up for you.

If you go for the NJ n Frankfurt or later it is probable you will take a train after midnight, unless you are already planning to use your pass for tomorrow DO NOT enter any train starting after 00.00 in your pass, this will activate tomorrow as a travel day.

I would suggest leaving the original time for the Koln - Zurich train active on your pass as that will be assigned to today.

If you do postpone travel until tomorrow due to the railway problems then as above DO NOT activate another day that you wouldn’t have used otherwise. Go to staff/ticket office wherever you end up and get them to authorise alternative connections.

And yes, if you cannot make the night train due to rail delays you are entitled to a refund.

Don't activate a new travel day. Just put your original plan in the app and travel with any available train.

I'd just stay on board until Düsseldorf. The pass reservation to Düsseldorf would be the same price and there aren't any passengers boarding at Cologne so nobody will claim your seat.

Yes you are entitled to a refund in such a case.

Just to add an update for you.

The Amsterdam - Frankfurt ICE251 has now been cancelled for Koln Hbf and given a stop in Koln-Ehrenfeld instead listed as 21.40 but currently timed as 22.10.

Thanks everyone for your very efficient help. I decided to cancel my journey to Zurich for tonight and stay in Brussels tonight as the homeowner I’d agreed to sit for managed to find someone else to look after his pets.

I’m going to get a refund for the reservations and go to Cologne instead for the weekend. There may be some issues with the flooding, but will be a lot easier to work around than getting all the way to Zurich.