Officials said that the dead included 15 men and three woman with about 80 people injured, 20 of them seriously.
Tim Friend, Al Jazeera's correspondent in Buizingen, said: "The rescue workers are now winding down for the night and I understand they are now pretty confident everyone has been accounted for.
"The investigators are now really trying to find out what went wrong, this point about a red signal being crossed has been raised.
"It's been raised by a regional official, so one assumes it has some substance, but it's by no means confirmed."
The fate of the two drivers was not immediately known, and officials said they were having difficulty identifying some of the victims.
The trains, carrying a total of about 300 passengers, collided in light snow just outside a station.
"When we came out we saw dead bodies lying next to the tracks, some mutilated," Patricia Lallemand, who was in a middle car of one train, said.
The impact peeled away the front of one train carriage and threw at least one other off the tracks, severing the limbs of some passengers, witnesses and officials said.
"It was a nightmare," Christian Wampach, 47, said after medical workers bandaged his head at a sports complex where the less seriously injured were treated.
|One of the front cars appeared to have careened across the tracks [Reuters]
Badly hurt victims were taken to 14 hospitals in the Brussels area, and the Red Cross appealed for blood donations.
"We were thrown about for about 15 seconds. There were a number of people injured in my car, but I think all the dead were in the first car," Wampach, who was in the third car of a Brussels-bound train, said.
One engine was thrust high into the air and snapped overhead power lines.
One of the front cars appeared to have careened across the tracks, demolishing a small maintenance shed next to the rail line.
A high concrete wall around the train yard seemed to have kept debris from hitting nearby houses.
Herman van Rompuy, president of the European Council, expressed his "shock and sorrow" over the accident.
King Baudouin and Yves Leterme, the country's prime minister, who cancelled a trip to Kosovo moments after landing in Pristina, have visited the crash site.
Eurostar cancelled its service from London to the Belgian capital for Monday and advised passengers the line was likely to remain closed on Tuesday.
It was the first serious Belgian train accident since March 28, 2001, when eight people died when a crowded train plowed into an empty train driving on the wrong tracks.