II. Summary Judgment on the Limitations Period

12 Christiansen also argues that the trial court erred in determining that the limitations period on his claim had run. Under FELA, "[n]o action shall be maintained . . . unless commenced within three years from the day the cause of action accrued." 45 U.S.C. § 56. In cases of latent injury, courts apply a "discovery rule," under which the cause of action accrues only when "'the plaintiff knows or has reason to know of the existence and cause of the injury which is the basis of his action.'" Matson v. Burlington N. Santa Fe R.R., 240 F.3d 1233, 1235 (10th Cir. 2001) (citation omitted).

Because the limitations period operates as an affirmative defense, the defendant has the initial burden of proving that the limitations period has run. See Koch v. Shell Oil Co., 52 F.3d 878, 880 (10th Cir. 1995) ("Because statutes of limitation and repose must be raised as affirmative defenses, [defendants] have the burden of proof on the issue of whether plaintiff's action was timely filed).

Accordingly, in the present case where Christiansen filed his action on January 9, 2002, we may affirm the grant of summary judgment only if Union Pacific has shown that no genuine issues of material fact exist to establish that Christiansen knew or had reason to know of his injury and its cause on or before January 8, 1999.