Curiously backwards. That's one way of reframing a disadvantage as an advantage. The train connection seems to be 3h15m to 3h30m from Neuss train station to FOSDEM. A single connection for the long-distance train in Cologne, the rest is local public transport within Brussels.
(The OP may been /s without me realizing.)
Being flexible with DB is expensive. Getting somewhere at all is generally cheap. Getting somewhere at a reasonable time is usually ok~ish priced. But being able to just take any train? €€€
https://belgiantrain.be for finding trains and tickets to/from the nearest station, Etterbeek (or use another station if you want to take the tram, where you just swipe a bank card). The ticket is valid for any train going to your destination. For those <26yo, the price is discounted. Welkenraedt is an intercity station with free parking that goes directly to Brussels, in case that happens to be near to someone reading this
Same with the Netherlands. Sadly no intercity stations have free parking but Nuth is on the path north and the highway exit basically ends in its parking lot. After a few stops you can switch to an intercity to Amsterdam
Maastricht Noord has a much bigger free parking, in case this part of the Netherlands is how you want to start your journey up north.
Looking at the track layout in OSM, I'm very surprised that the regional train from Roermond literally cannot reach the platform and passes within like 1-2 meters of the stop. That means that 99% of destinations in NL require you to first take a train into Maastricht and then backtrack (pay double) for the path north. Not sure I'd recommend that over Bunde (3km further, also free parking) unless your destination is one of the tiny towns towards Heerlen. Even if you want to hop onto the tri-country train (BE-NL-DE), which stops at the previous and next stations (Maastricht and Meerssen) and could simply stop at this platform, Maastricht Noord is useless to you O.o who is that rich person living somewhere along this track who funded that incapacitated station such that they can be its only user? xD
Edit: you are correct, btw! https://hn.algolia.com/?query="nuth"&type=comment
But, you're only a few minutes from Maastricht, so that shouldn't add too much time, and you get to get a good seat, since it's the start of the IC journey ;)
But outside of these hours the car is simply much more convenient. I lived in Brussels for 42 years and did everything that wasn't walking distance by car (very mostly in the pre- Uber days). You simply know where the parking spots are and it's too convenient to have your own car when you come out of the restaurant, without to have to worry about the last bus / last tram / getting mugged.
TFA's author went up early in the morning: he's dodging traffic jams.
For example FOSDEM if I'm concerned there'd be no spot? I'd park on the other side of the Bois de la Cambre and then walk to the campus.
Bicycling? It's nice when you don't have a nice bicycle. Otherwise it's gone in 60 seconds. I also don't see many people bicycling when the weather is bad and, well, let's get real: it rains a huge freaking lot in Brussels.
P.S: FOSDEM is happening in the Ixelles district, adjacent to the Uccle district (the Bois de la Cambre is on both districts). These are the two poshest, classiest, most expensive districts of Brussels with very few high-rises and very few soviet-style buildings with lots of apartments (these exists but in other districts). It's as if FOSDEM was taking place in Beverly Hills. In these posh areas there are parking spots.
As they say, there's no such thing as bad weather, only bad clothing. The weather in the Netherlands is not very different and it doesn't stop them.
Also the public transportation is by far the best of the country, but that doesn't say a lot.