That will depend on there being railway stations in Germany with spare platforms that can be walled off and space for UK immigration/customs facilities. If I recall correctly, the ones close enough to be within a competitively short journey from the UK (Cologne, Frankfurt and such) don’t have this kind of space, and the nearest ones that could be used are inconveniently far away.
The other option is to march everyone off the train at Calais and do immigration there, but that would add an extra hour or two to the journey, along with considerable annoyance.
That will depend on there being railway stations in Germany with spare platforms that can be walled off and space for UK immigration/customs facilities. If I recall correctly, the ones close enough to be within a competitively short journey from the UK (Cologne, Frankfurt and such) don’t have this kind of space, and the nearest ones that could be used are inconveniently far away.
The other option is to march everyone off the train at Calais and do immigration there, but that would add an extra hour or two to the journey, along with considerable annoyance.