Colo. Rev. Stat. § 40-27-104
Any railway company or corporation or the assignee or lessee thereof whose road is enclosed with good and sufficient fences, gates, and cattle guards, as provided in section 40-27-102, capable of keeping such animals from being upon such road may plead and prove the same as a defense to any action under sections 40-27-102 to 40-27-113 for any killing, damaging, or injury to such animals occurring within such enclosure; but such plea or fact shall not be held to preclude the owner or his agent from showing that such killing, damage, or injury was caused by the negligence or carelessness of such railway company or corporation or the assignee or lessee thereof, for the purpose of showing liability notwithstanding such fencing.
C.R.S. § 40-27-104