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.