I think you are on the right track. The problem for English speakers is that although Что/Какой roughly corresponds to What/Which, the Russian and English versions do not overlap cleanly. You found a good example:Originally Posted by Pretty Butterfly
Which train are you taking?
What train are you taking?
Both are correct in English, but you can only use какой in Russian.
What train? = какой
What day? = какой
What color? = какой
What <noun>? = какой
What is usually что, but when you say "what" in the sense of "which one of <noun>", it's какой. Basically, if you can use both what/which in English, use какой in Russian. After some practice, you'll develop a feel for this.
Note: "What time is it?" is not a good example for this area of grammar because that question is rendered in a totally different way in Russian: Сколько времени?
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Now I have a question. How would I translate a "what" question with a definite article: (ex) "What is the plan?" I'm not asking which because there can only be one, but I want to know what the nature of it is. Что такое?