Domestic Belgian trains don’t need reservations. Fill out the stations you leave and arrive the day you want to travel, generate the QR-code and hup. You can board with a valid ticket.

Don’t rely on the rail planner app for real time informations. It doesn’t update often enough. You might miss important information (because of e.g. engineering works or any other update in the schedule).

More real time info for the Belgian railways is to be found here: https://www.belgiantrain.be/en or on the NMBS/SNCB app.

There’s plenty of trains from Brussels to Antwerp. About 3 faster IC every hour, supplemented by lots of slower S-trains.

If you go out of Belgium, avoid Thalys. It’s reservations are ridicoulously expensive for pass holders. Use the hourly IC to the Netherlands, or ordinary TGV’s to France (or go to Lille) or ICE to Germany (optional reservations).

Neither the rail planner app nor the Eurail website should be used to plan. They don't get updates frequently enough to be reliable. The best is to use national websites or apps; for international journeys, the DB website or DB Navigator app is a good choice.

In this case, the new Belgian timetable (valid from 11 December) is not available in the rail planner app yet. It will probably be included in the next update. There is no need though to add your journeys until shortly before travelling and between Brussels and Antwerp, you don't need any reservations.

For Belgium, plan journeys on www.belgiantrain.be.

Thank you for the response! Since it doesn’t need reservations, is having a Global Pass enough as a ticket? Do we just show it at the entrace since we can’t generate a travel day on the app?

When do you plan to take the train @altcor ?@altcor ? And where? 

Just add the train you want to take in "My Trip" in the rail planner app, just a bit before you leave. The moment you see your train at the platform, tap on the ledger to generate the QR code in your pass and board. The pass (QR code) for that day is your valid ticket. With that, you can board any train that doesn't need a reservation. 

No need to show up in advance for a domestic train. 

The Belgian railway network is very busy in Brussels, especially Brussels Central. That can be a bit confusing since trains follow each other every 2-3 minutes on every platform. Just pay attention to the digital platform boards (in Brussels One side is in Dutch, the other in French) and announcements. (Antwerp = Anvers = Antwerpen; Brussels = Bruxelles = Brussel).

But in essence it's super simple, no need to stress about it, no need to be there hours before the train leaves. There are no ticket barriers or any other advance check. Trains enough in Belgium and they're extremely easy to use. Show up, have your ticket or pass, step in and enjoy your trip. 

Ah, what do you mean by the QR code being the ticket? If there is no train reservation, then the ticket won’t generate? Is there another way to get this? Thanks again for the response!

Did you check this?

You probably mean by “to reserve” that you add them in “My Trip”. In your case, you can’t do that just yet. You just have to wait until the new schedule is loaded in the app. December 11th there’s a Europe wide, new train schedule. Belgium was a bit late with making it public this year, so it’s not loaded in your app yet. It will be in a couple of weeks. The rail planner app doesn’t update often.

When it’s in the app, you can add the train you want to take (the reservation free one) in your trip. There you’ll see a ledger, which you can app to activate your travel day and QR-code for the day. That code is your ticket which you’ll need to show to the ticket inspector.

When do you plan to travel?

Some useful information, all credit to @AnnaB :)

Here is some useful information from the experienced travellers in the Community regarding both planning, reservations and activation of pass and travel days. 

 Planning

The rail planner is normally not up to date, as it only is updated once a month, so to be sure of the time table you better check the timetable and availability on the websites of the national railways. The bigger national railways, like DB (Germany) SBB (Switzerland) and ÖBB (Austria) cover several countries. 

 Reservations 

Mabuhay. Yes, that QR-code or the day [plus the trains you actually use are in fact the ´ticket´=allowance to travel. There are NO checks before, every now and then-or not on short trips, an inspector comes by and checks anyones tickets-that you show that code.

SOME trains-mostly INTERNational require a real RESvation=you get an allotted seat with date/time/car+seat nr. In fact that will very often not even be checked-you may have to show it if some dumb other person has taken your seat, but in general in such cases it is also marked somewhere on/above the seat.

Confusion here often arises as most from far over the oceans tend to use only the long-distance INternat. expresses and tend to think it all works like that.

In fact; if this the only trip to make that day-then a pass is very, very bad value-a normal ticket (it may even be discounted for some of you as ju/se-nior) is much less as 10€.

You are confusing tickets with reservations.

Your pass is your ticket to travel on most trains in Europe. It works by adding trains you intend to travel on into your Trip (on the app). When you are about to board you transfer the train from your trip into your now activated pass.

For the first train of the day you transfer to your pass and it will create a travel day. If you then click “Show ticket” in the travel day it will display a QR code and the details of the trains you add for that travel day. So as you transfer trains for that day by moving the slider they are added to the list in your ticket. The QR code remains the same all day.

That is all you need to do to travel on any train (except if reservations are mandatory). Some trains have optional reservations but that is a choice for you and doesn’t need presenting to travel, only to find your guaranteed seat.

You can add or  delete trains in your pass/trip at any time and as many times as you want before boarding, so if an earlier train is available you simply delete the planned train and add the new one.

Reservations for seats/sleepers are totally separate from your pass, whether they be mandatory or optional and have to be sourced from either Interrail reservation service or national operator sites, and there are plenty of threads and guides in the community.
 

So if you are boarding a train with a reserved seat you will need both your QR code and train details (your ticket) and a copy of your separate reservation (usually a pdf or a printed copy).