I had to try the tren turistico out of curiosity.
It is a road train with carriages which does an interesting route of Granada.
There are free headphones with commentary in several languages.
It is a hop on hop off service which means after paying for the ticket you can get on and off as many times as you like.
It is very touristy and it makes you feel like an obvious tourist.
It is a little bit expensive and it would be cheaper to get in a taxi and do the same route or just get on one of the normal minibuses.
However it is a pleasant way to spend some time in Granada and could be fun for a lot of people.
Click here for more information. http://granadainfo.com/alhambra/tren.htm