"That train is slow, isn't it?"
Actually it isn't correct. What you wrote means "trains are slow". The English sentence specifically states "that train".
Of xourse different contexts could change the required words, but we're not given a context so we should just stick to the most obvious and literal translation where possible. (While still having it sound natural in Japanese).
i would highly suggest to anyone who wants to learn how to speak the language take the time to type out all the answers. (with the exception of the listening parts, they are to buggy.) It is a lot harder and will take longer but it gives you a really solid grasp on pronunciation and senescence structure.
Note that you wrote Asoi (あそい), while it is Osoi (おそい)
As for your question, you may check the following link to see the 2 kanjis suggested for おそい (depending on the meaning): https://en.wiktionary.org/wiki/%E3%81%8A%E3%81%9D%E3%81%84#Japanese