I can answer “What risk is there on the ICE to not getting a seat reservation?” for the Dutch part, as I take this train every work day, without reservation.
There is an electronic display above each pair of seats that says on which stretch the seat is reserved, e.g. if it says “Utrecht – Frankfurt” and you sit there when the train is in Utrecht, you are likely to be asked to move to another seat. Many of these people do show up (but not all of them). There are also 6-seat rooms that have the displays besides the doors.
There are also seats that say “Ggf. freigeben”. It means that the seat may be reserved, it was available for “Express Reservierung” (don’t ask me how to do an express reservation). In practice you won’t often have to stand up if you sit here, but it does happen.
Many seats will also show no reservation, so it would be best to pick one of those.
Most of the time, there is enough place to sit, so a reservation isn’t really necessary. On some times and some days the train may be very full, mostly due to inside-Netherlands commuters like me (we leave in Arnhem at the latest), and on some moments like the start of the weekend due to actual international travellers. Then you’re unlucky and will have to stand, but luckily the Bistro serves drinks…
After Arnhem, the train becomes much quieter. I don’t know if it becomes busier again after Duisburg.
You have to pay a surcharge to use the ICE trains within the Netherlands, you do not need to hold a reservation, an international ticket covers your surcharge. See this page.
A good site for all train travel questions, in Europe and the rest of the world, is the site of the Man in Seat Sixty-One, here on the how to make reservations with a rail pass (but that also works when you have a train ticket already.)
If you have a rail pass, either Eurail or InterRail you do not need to pay the surcharge on the ICE trains within the Netherlands, see this page for Eurail or InterRail and go down till you see the paragraph about the Netherlands. Here a quote of the Eurail pass.
Amsterdam to Germany (Berlin, Cologne, Frankfurt & so on) by IC or ICE train: No supplement, reservation optional.
When using the Eurail pass in Germany you can use most IC trains without having to pay for a reservations. There are a few trains which do require a reservation, as explained on this page, in the Germany section, prices in the quote current in 2015.
Using a Eurail pass in Germany is easy, as reservation is optional not mandatory on almost all German domestic daytime trains and there are no supplements to pay, even on fast InterCity (IC) or high-speed InterCityExpress (ICE) trains. So you can simply hop on any train, find an empty seat and show your pass when asked.
The only exceptions are a tiny handful of ICE Sprinter trains aimed at the business market, marked in the timetable as ‘reservation obligatory’, on which a seat reservation is mandatory for €11.50 in 2nd class, €16.50 in 1st class.
You can easily find which trains have a compulsory reservation if you check your intended train on the German rail planner.
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