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.

Really. Crossing borders within the Schengen-zone is trivial and goes unnoticed for most travellers. You can even cross multiple borders in your sleep on the night trains.

It's so seamless that I was racing someone through Belgium and only realised I'd entered the Netherlands when I got a text telling me I was on a Dutch phone network!

By car you'd notice, but that's mostly because suddenly the car goes quiet upon crossing over onto the smooth asphalt of the Dutch snelwegen. (Despite being neighbouring countries, the contrasts are often interesting.)

I noticed the massive reduction in cobbled roads on leaving Belgium!

The problems crossing the border in Europe have nothing to do with the physical crossing. the Schengen zone is nice for that, but only solves one problem.

Often there is no border crossing at all where there should be one. Often nobody knows how to sell you a ticket for a trip that should be possible (or they will only do it with stupid restrictions). If you can find a train it may be a slow speed train despite this being trips where high speed is most useful. The trains may not be on time and you miss a connection.

there is a lot to improve about trains in Europe.

This is true. And sadly the big state monopolies have been lobbying against the airline like system of bookings. They don't want to be responsable for connection like airlines are. And they don't want to compete on a level bases either. They don't want open ticketing information.

The EU approach to rail improvement is incredibly flawed.

And all the different monopolies have their own dumb as opinion about things. French system is run by a bunch of airline manager idiots. German can't make their IT work for shit. And so on.

I don't have experience with InterRail, but the bad experience often starts with buying a ticket.

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."

Oh that's nothing. 5 years ago DB couldn't sell you all tickets within Germany (if you didn't cross a Verkehrsverbund boundary the local train company could refuse to license that right to DB). So yeah. Improving ;)

I am pretty sure that is still the case in some areas. DB Navigator never covered all local tariffs.

The trains exist, but they are not as useful as in country trains. Just getting a ticket can be tricky as not everyone will sell them. Most train systems assume you are going to the major city in their country, (Paris) and so the routes do not always make sense for other trips. One train per day (or less) is not good service. Cross border makes it much more likely you have to transfer to a different train and the poor on-time performance of some nations becomes a problem.

There are not border controls which is nice. However there are a lot of other things that can go wrong. Some routes work much better than others. Europe is a big place with lots of different trains systems if you only went on the good routes you won't have a problem, but there are a lot of routes that are not as good - and many more routes that should exist but do not at all.

It is a mixed bag, but definitely improving. Some experiences: From Holland, It was impossible to do an online booking a ticket to Italy. Only possible via phone, or as separate legs. For long journeys with changeovers watch out that you don’t get stuck halfway due to a missed connection. From NL, London, France, Germany is easily reachable, as well as Barcelona. I took a sleeper train to Stockholm last Christmas, we had delays, but made it in time. In general if you have a long distance leg, it’s best to have it at the start of your journey so you don’t miss it.

Railways do have single points of failure. A single disruption on a key piece of track can disrupt service for hours, even days. This is less of an issue with air travel, there’s usually other airlines, and alternative routes.

As a Czech person I have never npticed language barriers reducing travel (or business), if anything they increase it by making it more exciting.

This is not really true. There are high speed trains operated by Italy that start in the center of Germany (Frankfurt) and end in Milano – crossing Switzerland. There are also trains that start in Germany end end in south France (Mannheim - Marseille). Super easy to purchase tickets.

French trains are fast if they aren’t striking.

French train speed is fast but velocity can be slow if you’re not going toward or away from Paris.

If you’re going from the northwest to the southwest, it can be faster to drive than the high speed train that will dogleg through Paris.

(It’s getting better though)

I had zero trouble visiting my german friends by train last november, departing from Paris. I didn't even have to leave my seat. I have no idea what you are trying to say.

Not only does English get you by in most big cities, a fair amount of European citizens are multilingual and often it’s the neighbouring countries that people speak the language of.

You'll bet by in English as a tourist, but you probably aren't getting a job with just English, and you're definitely not holding down a full social life either. Despite the EU's efforts at integration, Europe is still effectively separated by languages and cultures way beyond anything in North America.