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.