Getting from Lille to Brussels airport is relatively easy, since Lille is connected to Brussels by numerous high-speed trains (TGV, Thalys, EuroStar), so it actually takes <40 minutes to get to Brussels. Those trains arrive to the so-called Brussels-Midi train station, so you'll need to make a quick train change there for the local train that will take you (ride will be about 20 min.) directly to the airport. At the end your whole journey will be slightly more than an hour of time in total.
It's still too early to book train tickets for next September, but you can check app. timetables & fares on Belgium railways website -
b-europe.com, while when the time will come you will be able to book your ticket there as well
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