Earlier quoted context omitted.
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.
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.