"Transportation by train is often very fast."
Translation:Le transport par train est souvent très rapide.
The translation shown here is the one I used. The question had the word "rail" instead of "train" is the only difference. This is the first time in over a year that the word "train" has been translated to anything except "train" And from the above, it looks like my answer was correct. Pretty shoddy.
I don't need a dictionary to know that "le transport par train est très vite" is plain wrong: on peut aller vite, mais on peut pas être vite.
Wiktionary does mention such usage as 'familiar', which is enough on its own to rule it out on a website that's supposed to teach academy abiding French, but even then the word 'familier' feels like an understatement to me. Now I'm not a promoter of linguistic 'correctness' and I use all kinds of slang daily, and it might also depend on the region, but I still can't imagine anyone other than a toddler use vite as an adjective.