Yes, rein is a Scrabble word!
...and is worth 4 points in Scrabble, and 5 points in Words with Friends
rein
Verb
1. Control and direct with or as if by reins
2. Stop or slow up one's horse or oneself by or as if by pulling the reins
3. Stop or check by or as if by a pull at the reins
4. Keep in check
Noun
1. One of a pair of long straps (usually connected to the bit or the headpiece) used to control a horse
2. Any means of control
What Scrabble words can I make with the letters in rein?
3 Letter Words
2 Letter Words