The accident happened as the train was departing Leuven station at a slow speed heading towards the capital.
Emergency services were quick to arrive at the scene of the crash, which happened at 12:20 GMT on Saturday.
The damaged carriage could be seen on its side part way down a slope.
Services between Leuven and Brussels have been suspended, officials say.
Last June at least three people were killed and nine injured in a train crash in the town of Hermalle-sous-Huy, near Liege.
In February 2010, 18 people were killed when two passenger trains collided at Halle, south-west of the capital, Brussels.
At least one person has been killed and about 20 injured when a train carriage fell on to its side following a derailment close to the Belgian capital Brussels, railway officials say.