Cars are usually slower than high-speed trains. There was a discussion a while ago with a high-speed train connection between both:

Luxembourg and Belgium in 2016 signed a letter of intent, pledging to reduce travel time between the two capitals by around an hour to 2 hours and 7 minutes by 2023, but this target was quietly abandoned. Last year, Belgium said the works would take at least another five years to complete. The projected date has now been pushed back even further to 2028.

Works on the line are complicated, Gilkinet said, especially as numerous curves need to be straightened to allow trains to pick up more speed. The trip currently takes around 3 hours and 15 minutes.

https://delano.lu/article/delano_faster-lux-brussels-train-not-2028

Due to the higher flexibility of the car, the train needs to have a clear advantage (here, the time) to be chosen by passengers, especially with the quite high price for a return trip (similar to the issue of trains competing with planes)

I said "EU hub" as hub for the EU institutions, which are mostly located in both cities