[ATsch]

> modern powertrains traverse various electrification schemes without difficulty.

Indeed, however the majority of currently existing stock is only fitted for operation in at most one or two countries

> signaling systems are trending towards ETCS

"trending towards" is doing a lot of work here, ETCS rollout has been stalling for ages. New train sets are still being delivered today that do not really support it. Most countries have somewhat understandably not been in a hurry to make the huge investments required to replace their current, working systems with ETCS.

> trains that traverse incompatible signaling systems (eg the eurostar) simply carry a set of onboard signaling equipment for each standard.

Yes, which is extremely expensive and also requires full re-certification for every country, which means that in practice most trains are only certified for one or two countries at most.

None of these would be unsolvable with some more willpower of course, but they are still absolutely an issue today.

[emn13]

Exactly - the issue isn't that any of this is unsolvable; these problems are solved, even today. The issue is that those solutions aren't free, and that they do impose limitations on the quality of service.