high speed should be hyphened or shouldn't ?
Usually, the hyphen is optional in two-word adjectives. I like to use it, it adds clarity and eliminates ambiguity. Is there such a thing as a word adjective and only two of them have usually optional hyphens? (See sentence #1 above) Is there such a thing as a speed train that operates at different heights? When it is at street level it is a low speed train and when it is on elevated tracks it is a high speed train.
The test I like to use is to read the passage aloud. If you can say something two different ways like high speed-train or high-speed train, then add the hyphen.