> That is, if you happen to own a nice locomotive, and miraculously have unimpeded direct access to all railways, then this tool will be a useful planner.

By law, that already is the case across Europe - all you need is a commercial entity, a locomotive (or multiple) that support all the voltages and signalling/trackside security systems used on the route, carriages that are certified (freight cars usually are Europe-wide, passenger cars run under RIC) and operators that have "Streckenkunde" (=they know where signals, switches, street passes etc. are on a route).

There are a handful of people doing just that, in Germany Roland Sandkuhl / LokRapid has gotten incredibly famous after a TV show featuring him and his old-timer Class 219 locomotive has gone viral [1].

[1] https://www.ndr.de/fernsehen/sendungen/hallo_niedersachsen/R...