First thing : seat reservations are optional in Germany. They are free with a 1st class through ÖBB : add Interrail/Eurail as a discount and select one-way tickets.

Now for 21st July : there are engineering works around Cologne so the ICE has to divert around the city, hence the longer than usual journey time. ICE13 is indeed full in 1st class between Aachen and Frankfurt. It is a small train on a very popular route so yes it's not unusual. However that doesn't mean that you won't be able to board/stay on the train as reservations are optional.

My advice : board the train in Brussels and swap seats in Aachen if needed. You'll almost certainly find a seat on the train -> someone missed the train, doesn't want to travel anynore,…

In the very unlikely case of overcrowding yes they can ask people without reservation to leave the train. But there are multiple seats in 2nd class so just move there if that's the case. If whatever happens, take the RE to Köln Messe/Deutz

You can alter the journey on Rail Planner at any point.

Thanks! Last question, Do I add 2 journeys in the app Brussels - Aachen & Aachen - Frankfurt on same train? Or 1 journey Brussels - Frankfurt?

The reason I am asking, if I have to change plans after Aachen, with 1 journey active till 12:32, will app allow me add new trains and link it pass?

Also , looks like option 2 is not possible RE is only running from Duren with a bus connecting from Aachen.

The 21st of July is the Belgian national holiday and traditionally a date where a lot of people leave on holiday; This train is going to be busy already from Brussels in Belgium.

1. Overcrowding in the ICE can happen, and happened quite a lot last year on the Frankfurt Amsterdam/Brussels. They allow some people standing, but in case of big crowds they ask passengers without reservation to leave the train and take another one. Usually this happens in Cologne or Frankfurt on the busier days (fridays/weekends). But there are plenty of ICE-trains between the two cities, so no biggie to take a later one. The pass gives you that flexibility.
2. Should be okay to make that connection with the RE, as said before. A lot of ICE trains between Cologne and Frankfurt (although some leave in Cologne Hbf and some in Messe/Deutz, but they’re a short 2 min train ride away with very frequent S-bahn trains (all included in the pass).
3. Definitely an option, if you have a lot of luggage e.g. and don’t want to switch a lot. I’lld recommend you to do this, so you don’t need to switch trains (even though the ICE takes a bit longer than usual). Carriage 22 usually has usually a lot of free seats in 2nd class when leaving from Brussels.
4. Take another ICE and reserve the whole stretch if you’re schedule permits (although there seem to be less than usual, probably because of the engineering works.)

• 1 journey will do.
• Every train you take needs to be registered. If you switch trains, it needs to be another journey (Eurail/Interrail needs to know, for the distribution of the money to the train companies). This proces is easy and fast, and can be done in seconds, even just before boarding. We actually recommend to always add a journey, until just before boarding the train.
  In this case, I wouldn’t recommend to get out in Aachen, because of the Engineering works. Try to get at least to Cologne, before deciding to take another train (much more trains to Frankfurt).
• Than I refer to my previous post. Choose an other ICE where you still be able to make a direct connection for your seat reservation in 1st, or make a 2nd class reservation for the whole way Brussels-Frankfurt. Traffic works and changing to buses is always a nuisance. When you can avoid them, you should.

To add : the connection via Düren is possible as long as the ICE is on time →

- Bus Aachen - Düren 09:46 - 10:35

- RE Düren - Köln Messe/Deutz 10:45 - 11:26

But try to stay on board the ICE, it should be possible to find a seat. Make a reservation in 2nd class to be sure.

Thanks @BrendanDB for the great advice

Thanks a lot for the advice!

I will stay in the ICE and proceed to Frankfurt. Only get out if it’s overcrowded and passengers are requested. 

There’s usually more ICE but not on 21 July unfortunately, maybe due to Belgium National holiday.

Mainly due to the engineering works I think. Unfortunately they have to be done in summer... on one hand yes there are less people commuting but on the other hand there are more travellers/tourists !

Enjoy your trip !