Earlier quoted context omitted.
Europe mostly speaks a different language in every country, so the actual scale of Europe isn't the whole continent (or whole EU), but a single country. Sure you can travel to other countries, but there is a language barrier (though English is common enough to get by most places). Note that GP was talking about Germany with seeming no awareness that the situation is different in Switzerland, France, England and Spain…
There is daily trains between all the countries you named (at least those that share a border) most of them are no different than any other border crossing with exception of the trains going to and from England as you have to go through security for that one. I don't remember ever having a bad experience crossing a border when I was on InterRail some years back.
The amount of "we cannot sell you that ticket, we can't even tell you the price" I have gotten when checking international connections is wild. It is slowly improving (I just checked a particularly absurd example where Deutsche Bahn was unable to sell a ticket to switzerland through if it wasn't bought a long time in advance - seems at least they fixed that), but still... it's absurd if your train trip starts with "figure out how to get a ticket, because the train company won't tell you."