That would be el último tren acaba de dejar. This is literally "the last train finished leaving" but the idiom for a newly accomplished action is acabar de inf. See: http://www.studyspanish.com/lessons/acabarde.htm
The use of ya is restricted to completed actions, what in English is usually translated as "already", and colloquially as "now" (como: Él es ya un adulto, which is either "He is now an adult " or "He is already an adult").
Denied is a strange choice of words. It implies that you were not given something that you were entitled to. As mentioned several times, the word just is not appropriate here. To say that something has just happened in Spanish you use the verb acabar, and you would use the present, not the present perfect tense. El tren acaba de salir. The train (has) just left.
Interesting. I heard the ya clearly, but had to extrapolate the ha from salido. Of couse having two identical vowel sounds together in adjacent words does make the second one disappear, and if you have a or ha, that's the whole word. Try saying Un día a la vez with any fluency. It sounds like un día la vez.
Ya as now can be used in that manner. It is used when you want to say it has left 'like just this minute something has happened'. I suspect DL didn't accept it as is but perhaps had you put' just now' that would give a better sense of timing. I can argue with myself because I don't feel 'already' is any better in clarifying when the train left.
Just wondering why the translation of: "The last train already has left." was marked wrong. In English, either placement of the word "already" doesn't change the meaning of the sentence. Not sure why the translation to English can't be word-for-word in this case. "Ya"="already", "ha"="has", therefore, "already has" should be marked correct as well.
You are correct Duo tries hard to impress on students the difference in syntax for some of these words. They need you to be able to translate what you would naturally say to the correct Spanish syntax. But they are overly inflexible in cases like this where there are several options and a few are equally used. Of the options for placing already here, the only one that actually sounds slightly unnatural (although absolutely correct) would be Already the teain has left. But Duo does want users to learn that just because you can translate either way into English doesn't mean that you can translate either way into Spanish.
That's how many of us ne'er-do-wells say it in America. The use of "already" without present perfect was accepted in the earlier "Adverbs" lesson (further up the tree) when we originally learned "ya", much to the chagrin of many British duolingo users. Using "already" without present perfect is not accepted in this lesson, because we are learning to provide translations for the verb "haber" (to have), which is the central idea of this lesson... Now the British get their revenge :)
Well, this is a lesson in present perfect, so...
ha salido = has left (present perfect)
salió = left (preterite)
I've been speaking English all my life too but realize that some people speak differently than I do in various regions not only in other countries but my own.
According to the grammar resources that I consulted, both should be accepted. The position of already is quite flexible and can go almost anywhere.
I am not sure if you are asking about the English or the Spanish. In English it works, although often Duo doesn't accept all possible placements of this type of adverb. English is rather flexible, but Spanish is not. In this case I am a little surprised though, because splitting up the verb phrase is comm in English but never correct on Spanish and Duo often likes to highlight that fact. But I think that the staff just doesn't play with the translation a lot. They tend to use what is most natural for themselves and forget about how often people can vary it slightly and no one thinks twice.
In American speech we say it that way frequently, but we seldom write it that way, except in very informal writing. I am a college educated native speaker of American English and even I tend to do a slight double take when I see a contraction that has a concrete noun with an apostrophe s, and even a little longer when the contracted word is has and not is. Even subject pronoun contractions with has are somewhat uncommon. If it takes me a second take to recognize that contraction, it would be very confusing for all the users who are learning a third language from English as a second language. Obviously that would have something to do with where they learned English, but it is an unnecessary burden. When I first used Duo maybe 5 or 6 years ago they didn't like any contractions, but they have gotten better. But as long as Duo teaches so many languages only from English, they will always have to deal with people with with more intermediate language skills. They tend to currently accept most contractions with not and will, but even contractions that I do see more often like should've are often not accepted.
You would need to use the reflexive verb irse, not ir. Ir is just to go so ido would ve went. Irse is pretty synonymous to salir however. So the sentence would be El ultimo tren ya se ha ido. With the ya in there I think salir would be a more natural choice, but I could not say for sure.
The already is necessary to convey the meaning, but I hardly think that it rises to the level of redundancy. The issue here is that it is also not needed in the Spanish. You will hear it in both languages, although certainly more consistently in Spanish. Duo tries to be somewhat natural in translations, but it is important, since this is a Spanish lesson, not a elegant translation lesson, to translate all the words with a meaningful translation.