The train, bound for Brussels, completely jumped the track soon after leaving Leuven station.
One carriage tipped over onto its side while another portion of the train travelled down a small slope.
Two of the injured are in a serious condition. At least a dozen others have also been hurt.
At least one person has died following a train derailment in Belgium
The victim was not a passenger and officials believe they may have been a hiker.
A spokesperson said: "It is important to note that that person was not on the train.
"Whether it is about a hiker, trail runner or someone else did, we can not say."
At least one person has died in a train derailment
The train was carrying between 50 and 60 passengers at the time of the crash.
The accident occurred at around 12.20pm GMT (1.20pm local time) 16 miles east of the capital.
Services between Leuven and Brussels have been suspended, including some trains heading to the airport.
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