Customs declarations are usually made at the entry-point (at least with international air travel) - but I don't know how trains do that today (all my train travel was Schengen zone in the late 90s - just passport checks on the train, and only between some countries).
if i'm getting you correctly, it sounds logistically WAY harder. authorities on every train for a start, then not missing anyone, then when someone shouldn't be allowed to pass customs they're already on the train so you can't kick them off, etc.
that's my question, where?
the only places I've done that were between countries with very special travel relationships, e.g. schengen. you can travel those countries without a passport and often no border check at all, it's not the same as uk-netherlands
German police boarded the train at the first station in Germany and walked the train to check everyone's passport.
Why would failing a customs check cause you to be kicked off the train? Either your goods will be confiscated, or you'll be arrested etc.
It's easier for everyone if you deny entry rather than prosecuting for unlawful entry. If you can kick people off while they're still in a country they are presumed to be in lawfully, they can figure out their documents and come back later. If you arrest them in the destination country, they've got to figure that all out from immigration detention.
When you take a train from e.g. the Netherlands to Germany you're already within Schengen at the point of disembarkation.
The police might check your passport details, but you won't be "deported" back over the border you just crossed.
You might be deported from Schengen in its entirety, but then the fact that you just crossed a border isn't important, but e.g. that you overstayed a Schengen visa.
In cases where there is a border with a non-Schenhen country in Europe the train will stop approximately at the border, and you might be refused entry.
But for e.g. Austria to Germany the German police might board at the first German station after the bonder, and then walks the train as it's in motion. That's because it's a customs check.
The checks are at the point of embarkation before you get on the train. You go through French passport control in London and British passport control in Paris/Brussels/Amsterdam.
Ths UK has this minister who seems to really really really really really not want people sneaking into the UK, not even if it's a handful per year.
The two paragraphs could be causally connected. I don't know.
You're a braverman than I to say that.