Using Interrail and Eurail Passes in Belgium

As reservations aren't required on any train operated by the Belgian national rail company, NMBS/SNCB, you can hop on/off virtually any train in Belgium if you're travelling with a Eurail or InterRail Pass.
The only notable exceptions are the Thalys trains between Brussels/Bruxelles and both Antwerpen and Liege/Luik on which rail passes can't be used at all.

To get value for money out of your pass while in Belgium, it's a good idea to plan multi stop day trips - and to see more than one great city in a single day.
These feasible multi-city itineraries include...
(1) Antwerpen → Gent → Brugge → Antwerpen
(2) Bruxelles → Antwerpen → Gent → Bruxelles
(3) Bruxelles → Mons → Ieper/Ypres → Bruxelles
(4) Bruxelles → Gent → Brugge → Bruxelles

How InterRail and Eurail passes can be used on international trains to and from Belgium differs according to which service you will be travelling by:

You can travel between Belgium and Germany on the 'ICE International' trains at no additional charge.
So if you have a rail pass, target these trains and avoid the Thalys trains (see below) for journeys between Belgium and Germany.

Paying the OPTIONAL reservation fees of €5.30 if you have a 1st class pass, or €4 if you have a 2nd class pass, to guarantee a seat on these ICE International trains can seem comparatively expensive; particularly for the relatively short journeys between Bruxelles/Liege and Aachen/Koln.
Though if you want to be sure of a seat on the longer journeys between Bruxelles and Frankfurt, it can be worth looking up the train you're planning to take on DB - the German national railways booking service.
Carry out the steps required to book a ticket and it will warn you if a particular ICE departure is likely to be exceptionally busy.

Rail pass users have to pay comparatively expensive rail pass reservation fees/supplements on all routes by Thalys trains, so avoid them if possible.
If you do want to travel on a Thalys, ideally book at least couple of weeks ahead of your travel, as rail pass reservations on Thalys trains can sell out, as a limited number of reservations per departure will be available to rail pass users.

A major recent change is that the rail pass reservation fees on the Thalys trains are now the same price, whether you will be using First or Second class passes.
Though users of Second Class passes will have to travel in Standard Class on the Thalys trains.

In contrast First Class pass users will be offered a choice of seating, depending on availability at the time of booking.
If Premium Class is still available then that will be offered, if reservations are sold out in Premium Class you will be offered Comfort Class (1st class), but if Comfort Class is sold out and
Standard Class reservations are still available, you will only be offered seats in Standard Class.

The new fees (in either direction) are:
Paris <> Bruxelles/Antwerp/Liege = €25Paris<> Aachen/Koln/Dusseldorf/Essen = €30
Paris <> Rotterdam/Schiphol/Amsterdam= €30
Bruxelles/Antwerpen <> Rottterdam/Schiphol/Amsterdam = = €20
Bruxelles/Liege <> Aachen/Koln/Dusseldorf/Essen = €25

The reservations be booked for a fee of €2 per person on the Eurail/InterRail reservation service and bookings usually open up to 4 months ahead, so book in advance to maximise your choice of departures, rail pass reservations can sell out on the most popular trains, particularly in the summer.

You can also make reservations for journeys by Thalys trains on B-Europe.
Though in effect you will be paying a €4 fee for using B-Europe, as it only sells rail pass reservations as 'mobile tickets', so that you can download the reservations and store it on your phone, but it applies a surcharge for booking these.
So it's more expensive than using the reservation services provided by Eurail and InterRail, but reservations can be available to book on B-Europe for specific departures, when the Eurail and InterRail reservation services don't have them.

B-Europe will ask for pass numbers when booking, but it only accepts the numbers issued with paper passes.
If you have a mobile Eurail pass you can use this pass number generation service and InterRail mobile pass purchasers can used this pass number generation service; you enter your mobile pass number. so that you'll also then have a paper number.

You can avoid these Thalys reservation fees by making indirect journeys with multiple changes of train.

Seats reservations aren't available on the 'InterCity Brussels' trains - so you won't pay any extra to travel with a valid rail pass by these trains on these two routes:
(1) Brussels/Bruxelles - Mechelen - Antwerpen - Breda - Rotterdam - Schiphol - Amsterdam
(2) Brussels/Bruxelles - Mechelen - Antwerpen - Breda - Rotterdam - Den Haag/The Hague

So if you will be using a rail pass and travelling between Belgium and The Netherlands in either direction, target these trains and avoid the comparatively expensive rail pass reservation fees on the high speed Thalys trains - see below.

The Eurostar reservation fee is a flat rate of €30 in 2nd/standard class and €38.50 in Premiere/1st class; the 1st class rate can be exceptionally good value, particularly when the most heavily discounted tickets have sold out.
Though Belgian and UK residents can only use the reservation fees on the first and/or last days on which they will be using a pass, due to the InterRail validity T&Cs.

A limited number of reservations are available to rail pass users, so booking at the earliest opportunity is highly recommended, the most popular departures can 'sell out' more than a week in advance.

Reservations are usually available up to 6 months ahead and they can be booked for a fee of €2 per person on the Eurail/InterRail reservation service; the rail pass reservation link on the Eurostar website is now directed at the Eurail/InterRail reservation service.

If you opt to use the paper version, or now the mobile* version, of the Eurail/Interrail pass, Eurostar reservations can also be booked B-Europe up to 4 months ahead of the travel date.
*= Using B-Europe's reservation service had only been an option for users of paper passes, as the type of numbers used for mobile pass users are rejected by B-Europe
But new online services have been launched which enable mobile pass users to obtain the necessary paper pass numbers.
Eurail mobile pass purchasers can use this pass number generation service and InterRail mobile pass purchasers can this pass number generation service.

However, B-Europe will only sell the reservations a 'mobile tickets', which incur a €4 surcharge.
So booking with B-Europe is more expensive than using the Eurail and InterRail reservation services, but reservations can be available for a specific departure on B-Europe, when they are not available with Eurail or Eurail.

Either side of the Christmas and Easter holidays and when travelling in July and August it's not unknown for some departures to sell out more than a month ahead; and it's the most poplar departures which will sell out soonest.
If you will need to travel by a specific Eurostar departure, you'll to check that rail pass reservations are available for the train you need to take prior to buying a pass, and then book the reservations as soon as you have made a pass purchase and therefore have a pass number.
Check the availability on B-Europe and on the InterRail/Eurail websites.

Rail pass reservations have to be booked before boarding the TGV trains between Bruxelles/Brussels and cities in France, other than Paris* including Avignon, Lyon, Marseille and Montpellier.
Though the comparatively expensive rail pass reservation fees are 1st class = €30; 2nd class = €20.

The reservations be booked for a fee of €2 per person on the Eurail/InterRail reservation service after you have purchased a pass and bookings usually open up to 3 months ahead, so book in advance to maximise your choice of departures, rail pass reservations can sell out on the most popular trains, particularly in the summer.
It also seems that the reservations for some departures are permanently sold out, so if you encounter this scenario contact Eurail or InterRail.

The other booking options for rail pass users on these trains are:
(1) Book at a station in Belgium, ideally before your travel date, though you will be charged a booking fee per reservation.
(2) If you will be travelling through Germany prior to arriving in Belgium, you can book reservations for these trains without paying booking fees at the Reisezentrum travel desks at the major stations.

*Thalys provides the direct train services between Bruxelles and Paris.

The alternative to the direct TGV trains:

If your travel itinerary includes direct travel between Bruxelles/Brussels and one of those cities in France, south of Paris, the only viable alternative to taking these direct TGV trains, is usually to take a Thalys train to Paris and then to travel on from Paris by TGV trains.

The reservation fees on that combination of trains will be either:
- €30 or €40 if you have a 2nd class pass,
- €40 or €50 if you have a 1st class pass.
You will also have to buy a ticket for the RER trains between the stations in Paris because rail passes cannot be used on those trains.

So IF it's an option, taking these TGV Bruxelles-France trains will be cheaper AND you can also avoid having to make the transfer across Paris.