Where would they be going on E* Red? Nobody would take one from Paris to Brussels (it would be cheaper to just go direct to Brussels) and I expect that the one to Germany from Brussels would be quite small especially as those services on DB/NS from Brussels to Dortmund are likely way more frequent than E* which are more catered to passenger from Paris.Presumably Eurostar knows how many passengers from St. Pancras are continuing on ex-Thalys services