A helper, BNSF SD60M # 9210, jumped the tracks near the refueling area in Sheridan, Wyoming a few weeks ago. No serious damage or injuries. Also a week ago two helpers SD75M #8233 and SD 60M uncoupled going uphill at 19 m.ph. then slammed into one another and broke both couplers and rear catwalks, and a BSP coal porter was crumpled in the inciedent. Its been colling off around here during the night, possibly contracting the tracks?
More likely the recent jump in moisture + freeze/thaw playing h*ll on the subgrade which in turn creates rough surface. All you worry about with cold and rail is pull aparts and switch headaches.