Unlike the aerospace, whose vehicles have very publicized operating manuals(probably because their hobbyist sims are much more true-to-life and closer to study level) it is almost impossible to find legitimate manuals online when it comes to trains. For example, I was looking for the documentation of the GE ES44C4, but the only thing I could find was this: http://www.chartertoconductor.com/wp-content/uploads/2019/06/es44dc-om.pdf, which doesn't have anything that I already don't know, and the things that I was looking for(cab signaling systems, automatic brake system schematic and its release time) are not covered, I don't think a manual for a locomotive can be that short in real life as cars quite often do have longer manuals.
You can find older manuals online. For instance, railcar.co.uk has a collection of various British DMU-related material: https://www.railcar.co.uk/books/
I've found this manual for the DB BR 101. (In German, obsiously.) The contents page after all the initial blabla is on page 12.
mike.obrebski put together a nice collection of UK/US/German operating manuals: https://forums.dovetailgames.com/threads/mp15-dc-documentation.55028/#post-490342
