Not required per se but sometimes recommended.

On a busy Sunday you'll stand for sure on Brussels - Hamburg without reservations. You might find an empty seat or two but never 6 next to each other. The full journey costs 5.90€ pp on the DB website, incl. seat map. A good investment!

Note that I don't get the exact same times:

- ICE15 Brussels-Midi 10:25 or Nord 10:34 - Köln Hbf 12:15

- ICE2410 Köln Hbf - Hamburg-Harburg 13:11 - 17:01

You might be able to catch an earlier train in Cologne but that depends on DB's mood that day... at least your seats on ICE2410 would be guaranteed.

Otherwise plan a little break in the first place, there's a cool little Italian restaurant nearby haha. EDIT: sadly closed on weekends...

Recommended on Hamburg - Copenhagen year round, esp. with a group for 6. It's the only railway link to Scandinavia so it's kinda busy.

Not required/not possible on Copenhagen - Göteborg - Oslo.

Thank you so much for this! Took your advice and got the reservations. 
 

really appreciate all the help. What a great resource! 
 

peter

I did Copenhagen to Goteborg last week, the one connecting nicely with the train from Hamburg, with Oresundstag, and it was packed with people standing all the way. That operator does not do reservations so be prepared or look for alternatives.

thanks for the tip. what time of day did you get this train?

On Monday 5 August, departing CPH airport at 12:44.

That is a hint as well: get on that train at the starting point, which is CPH H.

In mid-September it should be better in my opinion. But yes board at the departure station so Copenhagen main station.

Great. this is fab. I had planned on boarding at København H- is that what you mean?

Thanks folks. I find this so helpful.

Peter