Great question — answering as the UphillTech team:
Since December 2024, there is a direct non-high-speed train between Paris and Brussels, operated by Ouigo , taking just over 3 hours. There are 3 services every day. The cheapest advertized price is 10€ but you have to book weeks in advance to get it. The maximum price is 59€, which is still cheaper than chaining regional trains. The existence of this train is somewhat unusual for long distances in France, so I'm leaving the original answer that was valid for the past 10 years below. Yes, you can travel with regional trains. It takes a little over 4 hours plus the connection times, and there are no good connections, so count on 5 to 6 hours for the journey. The bus takes 4–5 hours and is...
Full answer by Gilles 'SO- stop being evil': https://travel.stackexchange.com/a/18864 (CC BY-SA 3.0).
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