There's also a lot of other use cases! For example, if you have a train schedule with stations and times, you often aren't told how the train will navigate between those stations. In some countries it's straightforward but for instance SNCF TGV services in France sometimes go hours without stopping at a station, crossing a lot of distance. In that case a solution like this can help you find an educated best guess to show users on a map.

Reading your comment I also realized that something like this would also be quite useful in the eventuality of a grand war in Eastern Europe (let’s say) and of the sudden necessity that would arise at that point of sending big and heavy stuff (ordnance, tanks etc) by train from one part of the continent to the other. Even though I do hope that the European militaries already have this type of policies in place (and hopefully handling connections between several countries’ rail-systems).

> 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...