The Rock Island route turns away from the Joint Line, coming north from Colorado Springs (MP 609.6) at the wye, 1,860 ft. siding, and yard at Roswell (MP 607.3), single track, maximum speed 25 mph, heading east-northeast, making a flat crossing and junction with the other side of the Joint Line at Curtis, where it turns east-southeast, past a spur on the north side to the Keystone Mine, turning east and then very curvily east and climbing to the 6,880 ft. elevation Tip Top Summit, descending past the 600 ft. spur at Elsmere (MP 600.1), where a line heads south, and turning curvily north-northeast, past Shirley, the 800 ft. spur at Falcon (MP 591.3), where there was a flat crossing with the erstwhile original Colorado & Southern, and the 640 ft. spur at Peyton (MP 582.0).
The line turns curvily east-northeast, past the 640 ft. spur at Calhan (MP 570.8), Ramah, the 1,920 ft. spur at Simla (MP 556.3), and the 920 ft. spur at Matheson (MP 550.0), turning northeast past Resolis, and then east-southeast and finally just north of due east, to the flat crossing and junction with the UP at UP Crossing (MP 530.8), where Rock Island trains from Denver trail in on the north side, and the yard at Limon (MP 530.8).