You need to pay the fee if you want to enter or leave the railway station. So you don't need to pay it if you're changing between trains, but you do need to pay it if you want to change from a train to a bus.

You can of course try your luck with the station gates (the have barcode readers; try the Interrail barcode) and the staff (maybe they'll let you through but probably not) and only pay the fee at the airport if you can't get through. There's a ticket machine right next to the gates, also on the inside.

If you arrive by plane and use a bus to leave the airport, then you don't have to pay it. The bus station is open to all.

Depending on where you're going, you can also get a regional bus to another station and avoid the diabolo fee. There are bus lines in various directions, not just to Brussels.

What a pleasure you want to visit Belgium.

The fee of €5,70 is because off the “Diabolo”-tunnels that were recently build to access the airport, and these make tickets to the aiport quite pricy.

This fee applies to all travelles, wether your just a local traveller using regular tickets or someone on an Interrail-trip.

As rvdborgt already mentioned, this fee applies only if you wish to leave the station, if you just want to catch another train at Brussels Airport - Zaventem you don’t need to pay the fee.

I don’t know where your going to or coming from, but there are several options by bus:

• Central Brussels: you can use the MIVB/STIB busline 12 which goes to the NATO headquarter, Meiser, the European district and Troon/Trône nearby the royal palace, you can find the timetable here.
  In terms of ticketing for bus 12, when your going to Brussels you need a special “Go2City”-ticket for this bus, which costs €7 using a contactless card on the bus or €7,50 when buying a paper ticket at the ticket machines.
  When going to the airport, any MIVB/STIB ticket is accepted.
• Brussels North railway station: you can use several lines of De Lijn (272, 471, ...), timetables can be found on their site.
  On these buses all regular De Lijn tickets are accepted, but please note that due to the pandemic the front door is still blocked, which means no tickets can be payed using contactless since that machine is located at the front door.
• To Mechelen you can use lines 682 and 683, those are the fastest, there are other lines to Mechelen (like 282, 686, ...) but those are slower so your best option would be the 682 or 683.
  Ticket options are the same as for the lines to Brussels North.
• To Antwerp you also have the Airport Express from De Decker-Van Riet which runs every 2 hours and brings you to Antwerp in approx. 45min. for just €10.
  The stop at “Koningin Astridplein” is located right at the front of Antwerpen-Centraal station, so this bus would an interesting alternative to the train.

Note that all these buses will cost at least 2,50-most a little more and take considerable time. So the savings are not that big. Plus that due to covid there still is a cumbersome way to buy tickets-not on the bus itself. The direct airportbus from MIVB/STIB also cost more as the diabolo-supplmt.

If you really want to avoid it and have no heavy luggage, you can walk to station In the village (Zaventem) in around 25-30 mins ( a little over 2 kms, IF you know the way!).

BTW; in Espana=Spain nearly all the outer ends of raillines to airports also charge a higher fare as normal-urban, mostly also around 5€ (f.e. Barcelona, Madrid/metro, Valencia/metro). Also often in ITaly.