They (LGG and UPS) are now discussing on the eurocarex project (cargo via high speed train). The whole goal of LGG mgmt is to convince UPS to keep the Liège hub alive and running.
A cargo high speed train could link both airport in 40 minutes.
The high speed tracks are already existing. Thalys connects Liège and Cologne multiple times per day for passengers.
EURO CAREX : Cargo Rail Express