"That train is slow, isn't it?"
I don't know if it's less polite, but the English sentence does not say "that train" anywhere. If it would be anything, it should be "this train".
I remember the kanji but i dont remember how it is said, i wish there was a way to hear it after writing the sentence. I would rather learn how to speak japanese than to read it.
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.
It's like... おそい means 'slow' ex. 電車はおそい、while おそく means 'slowly' ex. 電車はおそくはしっている (train is going slowly). I'm not sure if you can say this that way (they would probably use 'ゆっくり') but you can see the difference.
It just another word form, and should be used in certain situations. Like for "write" in English, it can change to "writes", "writing", "written" .... . To form a negative with ない, you have to use おそく.
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