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.