What's the song used when previewing the next episode of Fairy Tail? At the end of each episode, there is a preview of the next episode. It also plays in the very beginning of episode 15 of the new Fairy Tail 2014, I think that makes it episode 190 in the total running.

What is the song that plays when they preview the next episode?
The track that you're referring to is a bit unclear.

The pre-2014 preview music is different from the 2014+ preview music, and the music at the very beginning of 2014 episode 15 (the track before the theme song itself) is different from those two.

However, I notice that the track that plays immediately after the theme song in 2014 episode 15 is the same one as the one that plays during the 2014+ previews.

If this is the track that you are referring to, then the name of the song is

"Fairy Tail Main Theme 2014 (FAIRY TAIL メインテーマ 2014)" 

(The Katakana as Romaji is "mein teimu" i.e. "main theme"). 

It is track 1 on disk 1 of the album "Fairy Tail Sound Collection" by Yasuharu Takanashi (the fifth original soundtrack for Fairy Tail if you don't count the movie soundtrack).

I believe that I have supplied enough information for you to look up the track. I am hesitant to link to a Youtube video because I have noticed that Fairy Tail OST videos get taken down due to copyright violations very quickly and frequently so I don't want to provide an invalid link.