[emn13]

Of course technical hurdles can be worked around - the issue is that those workaround often have limitations and consequences. Trains won't be as interchangeable as they might be. Not all trains and all routes will be chosen because they're ideal from the perspective of traveler's logistics, but to stick to technically more feasible routes. There's going to be cost involved in making trains support multiple systems. Electrical systems can convert between voltages, but doing so efficiently, cheaply and robustly isn't that easy, and they'll take space and weight in the train too.

All those factors reduce the overall efficiency of the network, and they help explain why it's so slow and costly to travel large distances by train - much more so than you'd naively expect given modern train speeds.

For instance, I just tried looking for a connection between Groningen,NL and Murcia, ES. The train connection takes 29-30 hours. Adding busses into the mix reduces that time to 23-24 hours. Avoiding high-speed rail entirely and using a car would take ~21h - despite the trains going much, much faster for long stretches of the trip. By contrast, if you take long trips within e.g. France or Germany countries, it's pretty common for the train to be the fastest option.

The interconnections just aren't great, and I bet part of that poor connectivity is due to the fragmentation in the standards involved.

When you assert that these hurdles aren't impossible to take you're of course correct - but that doesn't mean the hurdles are irrelevant either.