Passengers travelling from Brussels-Midi to Lille-Europe on Eurostar trains are not subject to U.K. Border control checks at Brussels-Midi, nor are they required to go through Schengen exit controls as they are traveling within the Schengen area.
They are however required to pass through security checks in a dedicated Intra-Schengen terminal at Brussels-Midi.
In addition these passengers are asked to board a segregated coach from the remainder of the train.
This is to ensure passengers who are travelling on an Intra-Schengen journey do not remain in the train bound for the UK without having gone through UK Border control checks and Schengen exit checks. On arrival in Lille-Europe, all Intra-Schengen passengers must disembark from this coach and exit from the platform.
In order to do this we have to section off the relevant coach on the train to allow these passengers to travel.