"C'est un train historique qui traverse l'Europe."
Translation:This is a historic train that crosses Europe.
It's both "an historic" or "a historic" depending on your English pronunciation. It's not necessarily an aspirated h.
For example check https://en.m.wiktionary.org/wiki/historic which lists /(h)ɪˈstɒɹɪk/ as the British pronunciation, where the important part is the (h), indicating there are two possible pronunciations.
Further down,there is a usage note saying that some people use "an historic", while other people use "a historic"
Although I was marked "correct" on this one, I would like to "correct" Duo. They advised me that it's, "This is A historic train" and what I typed was "This is AN historic (although I used "historical)" train . To stave off future "incorrects" in the future, you will find that I am in the right (that is if Duo will care to look into the matter).