There's a wealth of information on Wikipedia, but often buried in local language pages.
The English page version is often truncated to generalities.
For example, most of the main lines in France are best covered in pages like "Paris-Strasbourg Railway".
Just compare the detail given in the English and French pages:
https://en.wikipedia.org/wiki/Paris-Est–Strasbourg-Ville_railway
https://fr.wikipedia.org/wiki/Ligne_de_Paris-Est_à_Strasbourg-Ville
You can always use translate buttons to get a pidgin-English version (but don't expect technical details to translate well).
Most of the detailed route diagrams have links to connecting lines, so you can step from one line to another easily. Works particularly well in Germany.
The series of pages like "History of Railways in..." generally produces a good background for each country, with links to more detail.
For instance there's a chart in the page on Austrian railways which shows the way all the original railway companies merged into the state railway (kkStB).
https://en.wikipedia.org/wiki/Imperial_Royal_Austrian_State_Railways
Some countries are covered better than others.
German-speaking countries seem to do best, but it's harder to find comprehensive information for newer states like Poland and "Yugoslavia", where the railways were mostly built by previous empires, often more than one - Poland's railways were built by German, Austrian and Russian owners.
For maps, I find
http://www.bueker.net/trainspotting/maps.php very useful.
It has country maps and all sorts of local operational detail.
However I believe it has not been updated for a while.
For rolling stock, try
https://www.vagonweb.cz/razeni/vlak.php?zeme=CD&cislo=172&rok=2012&lang=en
It will give you train formations for all long-distance trains in Central Europe and further afield, and a host of operational detail.