Translation:The director hung a map of the country near the trains schedule.
We would never say "trains schedule" but "train schedule"
"trains' schedule" is grammatically correct, but weird
Agreed, when a word like train is suffixed with an 's' it implies only either possession or pluralization (or both). If the 's' is there, it must be one of these three to make any sense in English:  The schedule of the train (the train's schedule) or (the train schedule).  The schedule of the trains (the trains' schedule).  The schedules of the trains (the trains' schedules). There is no example that makes any good sense where "the trains schedule" can work in English, unless you are saying that the the trains schedule a route for themselves automatically via on-board computers. And while there are seldom any strict "always/never" rules in any language, this is one of those distinct situations one might cite as an example.
I wouldn't think it's enough to accept a correct translation if the system still recommends an incorrect one - but it's certainly the first priority. I'm using the tiles, so I don't have the option here of giving a correct translation (or the frustration of possibly having it rejected).
The Google N-gram viewer is not able to find "trains schedule"in English literature.
i got this one wrong on the first try because i used "a" instead of "the." correct me if i am wrong but russian doesn't use articles, so why are you being so precise with something that can be imprecise in english? noticing a few things too where an english speaker (me) will leave out "that" in a relative clause and get marked wrong even though if it is part of an object, we can leave it out.
Why was "principal" offered as a hover hint, and then marked wrong? I suspect it's actually an oversight, but it seems rude. Especially as a school seems a more likely place to be hanging maps and schedules than most executive offices. :)
A general note on hover hints: they should be helpful hints, not traps. If someone consults them, it's probably because they are genuinely stumped, or wondering which translations are going to be allowed. They should help the user move forward and learn.
OK, I do feel better knowing there isn't someone cackling at his computer. )))
As a user and as a software designer, I have to suggest there is some room for improvements in the heuristics (clearly the word "algorithm" would not be appropriate here). To start with, wouldn't it be better to generate the hints from the "rules" for accepting the user's translation?
It may be just because they are more similar to English, but Swedish and Spanish Duolingo seem almost always to have some form of an accepted word included amongst the hints, while Russian very often doesn't. It would seem that in these other languages, maybe the metadata includes whether the word is being used as a noun or a verb, and likely what case and gender are involved (since very often the top hint is the correct form). It would be incredibly useful if in the answer in the Russian version, metadata of gender (which of 5), case (which of 6 or 7), and tense (of how many?) could be listed below the words in the answer. It would answer some huge fraction of the questions in these discussion sections, and would seem to need to be present in the question generators, since there seems to be quite a lot of variety in nouns or verbs chosen at random for various questions.
I'm sure Duo has some utterly bizarre reason for rejecting it, but the most sensible reason would be the use of train's. If you are going to use a possessive here it should be plural: trains'
On the other hand, the expression is actually "train timetable" and then even though train is singular it will refer to the schedule of trains.
But your version is already better than the one offered by Duo.
Given that the point of this course is to learn Russian, a translation indicating understanding of the Russian but in non-idiomatic English ("trains schedule") is, I would say, preferable to one that would permit an incorrect understanding of the Russian (via not distinguishing genitive plural from genitive singular).
A typical adult US English speaker will not use "hanged" other than, potentially, to refer to hangings of people. I would agree that "hung" should be an acceptable informal variant to talk about execution by hanging. Confusing "hanged" for "hung" in standard American English would cause one to come across as sounding like a three-year-old. I apologize if this comes across as somewhat brusque, but it is simply the reality. Obviously I cannot speak to your local dialect, where it would seem the situation may differ, but you certainly do not speak for typical American English. As evidence to support this position, I reference the Corpus of Contemporary American English. "Hung" appears approximately 17 times as often as "hanged." I did not read the extracts for all 1136 occurrences of "hanged," but of the first 100 I think only two did not refer to an execution or suicide by hanging, and one of those was a metaphorical extension of that concept.
In a previous exercise in this same lesson we had расписание занятий, which DL correctly translated "class schedule." It could also have been translated "schedule of classes," but definitely not "classes schedule." Here we have расписание поездов," which can be translated "train schedule" or "schedule of trains," but definitely not "trains schedule."