ex: 01-..He had been training hard that year. It had been years of effort...
02-..He had trained hard that year. It had been years of effort..

Someone is telling a story about an athlete and the Olympic Games of 1992. Are both of them correct? Are there slightly differences?

Question

ex: 01-..He had been training hard that year. It had been years of effort...
               02-..He had trained hard that year.  It had been years of effort..

Someone is telling a story about an athlete and the Olympic Games of 1992. Are both of them correct? Are there slightly differences?

Answer

The first one is Past Perfect Continuous. It tells us that the action had been continuing up until an unspecified point during that year. Since the next sentence emphasizes of the continuity of the action, Past Perfect Continuous seems to be the better choice.

The second one is Past Perfect. It would be the better choice if you wanted to emphasize that the year was over (or nearly over) when something else happened, but that doesn't seem to be the case in your example.