> 
> How can you take advantage of this? A gratitude journal is an easy and
>  actually pretty fun way to keep your attention ***trained*** on what’s
>  important in life: all the beautiful, exciting and wonderful things
>  that surround youday to day but which you may have forggten to
>  appreciate.
> 
> 
> 

The above contents come from the book *The 21 Day of challenge happiness* 

As for the bold word, trained, is it a past participle? what is the grammatical facts of it?

According to my research, I think trained on is a past participle phrase which modifies attention. Is it true?