All 3 Uses

rein
in
The Whipping Boy
(Edited)

The coachman, looking just as old and rickety, held the reins of his two-horse team and cracked his whip in the air again.
p. 52.2

reins = straps attached to a bit in a horse's mouth that are used to control it
The hot-potato man pulled up on the reins.
p. 53.8
"Rein in your beast," he whispered stiffly.
p. 60.8 *

rein in = restrain

Definitions:

(1) (rein as in: to rein in their activities)
to restrain or control; or a means of control
The meaning of rein depends upon its context. For example:
- "keep a tight rein on the new employee," or "rein in a horse" -- to control or restrain
- "give the new employee free rein," or "give the horse full rein" -- do not restrain
- "the reins of government" -- means of control
- "the reins of the horse" -- leather straps used with a bit to control a horse (You might like to think of other senses of rein as being figurative derivations from this sense.)
(2) (meaning too rare to warrant focus)
to "give free rein" is not to restrain (not to exercise control over)