Train to (pre)metro transfer was asked in a PM and I checked current situation out. Astrid station has closed entrances due to construction works and there is a shorter route to Diamant station.
Just in case someone else is asking the same question:
Long distance intercity trains should arrive at -2.
The signs at the escalators tells that moving up takes you closer to the metro.
At the end of the escalators, you should see this on the left.
Follow the triangle pointing to.
Escalator to the (pre)metro.
At the end of this escalator, turn right.
It looks there like this.
Go to the escalator on the right hand side.
The signs on the walls looks like this.
2 and 6 is going to the Sportpaleis from this station (Diamant).