Gentoo doesn't "exist" because it is necessary to have an alternative to systemd. Gentoo is simply about choice and works with both openrc and systemd. It supported other inits to some degree as well im the past.
According to this page, it actually did stop at Troisdorf (though, that doesn't have to be correct). I don't see why they should have been able to stop at Neuwied but not any of the stations in between. Most of them are possibly too small, as the RE5 is quite long for a regional train, about 200m. The usual "RE" on this track, the RE8, is only about 110m max. Bonn-Beuel should have worked, though.
I haven't. The Nix language makes no sense to me and there is still nothing akin to useflags. I don't want to override a bunch of packages just to make my system not pull in (e.g.) UI libraries.
Atomics were introduced a while before `counters`, maybe you remember a custom implementation on top of that?