Black Butte (MP 345.2, el. 3,898 ft.) is the junction where the former SP Siskiyou line, now operated by the Central Oregon & Pacific (CORP), heads off to the west, leaving the UP (ex-SP) main line at its facilities that include the north end of the south siding and the south end of the 5,400 ft. north siding, at least one extra track, perhaps two, on the west side, and on the east side of the line is an operable water tank, still used into the 1990s when steam-powered excursions ran.
After setting off due west, leaving behind the UP CTC and entering Track Warrant Control, unsignaled, with a 20 mph speed limit, the line turns northwest and then west again before turning due north after about a mile. About three miles further north, the line turns west again into Weed (MP 348.4), where there is a 3,421 ft. siding, an industrial spur that used to be a line to Bolam on the Natron Cutoff curves away to the north, and the speed limit rises to 25 mph. There is a detector at MP 349.9, where the line turns north again, Edgewood, where the line turns generally northwest, although with many curves back and forth, a turn back to the north, a 5.343 ft. siding at Gazelle (MP 361.0), on the level floor of the Shasta Valley, where the line becomes dead straight and due north across the valley floor.
There is a detector at MP 363.3, a spur at Grenada (MP 369.1), the straight segment ends with a curve to the north-northwest, and then one back north again, there are Yard Limits at Montague (MP 375.5), where the Yreka Western line heads off to the west, followed by a return to TWC as the line turns northeast and then north again at Snowden, a turn to the north-northeast, a detector at MP 384.8, a turn north-northwest before Ager, a bridge across the Klamath River followed by a turn to the west, following the north bank of the river, and a turn to the north-northwest again to the 3,583 ft. siding at Hornbrook (MP 393.1), where the speed limit falls to 20 mph again as the line enters the beginnings of the Siskiyou Mountains.
The line turns north, northeast, west, north past a detector at MP398.0, generally west with some curves, north again to a detector at MP 401.4, where it turns west and then northwest through Hilt (MP401.8), and the speed limit drops to 12 mph for the main passage through the mountains. The line turns just east of north as it crosses from California into Oregon, there is a detector at MP403.3, the line turns northeast, north, west, north past a detector at MP 407.5, and northeast through 3,107 ft. Tunnel 13 (MP 411.3) to the 4,588 ft. siding at Siskiyou (MP 412.2), the line's summit. The speed limit drops to 10 mph for the descent to the north, as the line turns north through 1,192 ft. Tunnel 14 (MP 414.6), and north-northwest through 258 ft. Tunnel 15 (MP 415.2), and then makes a clockwise horseshoe curve past a detector at MP 416.6 back to the south-southwest as it descends the mountainside, before making a counter-clockwise horseshoe curve back to the north-northwest to Steinman (MP 419.1).
Continuing north-northwest, the line passes a detector at MP 423.3, a spur at Belleview (MP 426.2), where the line turns northwest. The speed limit rises to 20 mph and the TWC is accompanied by Automatic Block Signals before the 5,875 ft. siding at Ashland (MP 429.1), and the speed limit rises to 25 mph again thereafter as the line heads west-northwest through Talent (MP 434.6), and then northwest again through Phoenix (MP 437.0), Gas Works (MP 438.4), Voorhies (MP 438.8), where control becomes ABS only, and Kane (MP 440.7), where the speed limit drops to 20 mph and the yard begins, to Medford (MP 441.8), where the speed limit is 10 mph as the yard continues.
On the north side of Medford (MP 441.8), a line to Jacksonville once headed west-southwest, and a line to White City once headed north, as the yard ends and the speed limit increases to 25 mph again. There is a detector at MP 445.0, Central Point at MP 445.7, a 3,858 ft. siding at Tolo (MP 450.2), where the branch to White City head due east and TWC resumes along with the ABS, and a detector at MP 452.8 before the line enters the curving section climbing westward to Grants Pass. The line turns northeast, north-northwest, west-southwest to Gold Hill (MP 457.2), south, southwest, west-southwest, west past the spur at Rock Point (MP 459.4), west-southwest, southwest, northwest to a detector at MP 463.0, north-northwest to the 2,579 ft. siding at Rogue River (MP 464.9), southwest, west-northwest with some curves, west southwest to Bulb (MP 472.2), beginning of a yard, and northwest to Grants Pass (MP 473.9), end of that yard, where a line to Waters Creek once headed west.
The line continues curvily northwestward to a detector at MP 477.3, and then turns north, northwest, north again, northwest again, north-northwest past Merlin (MP 482.5), where the speed limit falls to 10 mph, and a detector at MP 482.6, northeast with some curves and then north to the 4,200 ft. siding at Hugo (MP 487.4). The speed limit rises to 20 mph, as the line turns west-southwest, and then north-northeast through 2,105 ft. Tunnel 9 (MP 490.6), past Tunnel 9 Spur and a detector at MP 492.0, northwest past a 3,366 ft. siding at Leland, north-northeast on another 10 mph segment, northeast past a detector at MP 498.7, and east-northeast to Wolf Creek (MP 502.0), where the speed limit is 25 mph for a short distance. The speed limit drops to 10 mph again as the line heads east and then makes a tight counter-clockwise horseshoe back to the west before turning northwest through 2,819 ft. Tunnel 8 (MP 505.2), north-northeast, west-northwest past the 3,100 ft. siding at Glendale (MP 507.9), entering dark (unsignalled) TWC territory once more, north-northwest and then southwest, past a detector at MP 509.8, where the speed limit rises to 20 mph and the line turns west, with some curves.
The line turns northwest again, with many curves, as the speed limit once again falls to 10 mph, and passes through 128 ft. Tunnel 7 (MP 514.1), 516 ft. Tunnel 6 (MP 514.7), 341 ft. Tunnel 5 (MP 515.7), and 323 ft. Tunnel 4 (MP 516.0). There is a detector at MP 517.0, 430 ft. Tunnel 3 (MP 518.6), 432 ft. Tunnel 2 (MP 521.0), a detector at MP 522.3, where the speed limit rises to 20 mph, Dothan (West Fork), a turn north and then curvily northeast, a bend southeast, northeast past the location of Tunnel 1, and northwest, then a turn to a curvy east-northeast, a sharp turn due south, northeast to a detector at MP 538.0, southeast, and then definitively east-northeast past Cornutt (MP 540.3), where ABS begins again, and a detector at MP 542.7, northeast past the 3,080 ft. siding at Riddle (MP 544.2), east-northeast again past Weaver (MP 549.3) to Myrtle Creek (MP 550.4), northwest, west, northwest again and then north to the 4,461 ft. siding at Dole (MP 554.9), and west and then just west of north to Round Prairie (MP 557.3).
The line curves around to the southwest, and then northwest to Dillard (MP 562.0), where TWC ends but ABS remains, east past a detector at MP 563.0, and north along the west side of The South Umpqua River. The river turns away west-northwest and the line northeast, and the line then turns north through Green (MP 567.7), the river comes alongside to the west again as both curve north-northeast, northwest, northeast through Oaks, and north-northeast into Roseburg (MP 572.6), where there is a yard. North of Roseburg, TWC rejoins the ABS, there is a spur off to the east, the line turns northwest and then north again as the river heads away west, there is a detector at MP 575.7, Winchester (MP 577.8), a curve north-northwest, a bridge across the North Umpqua River, a curve north through Akin (MP 579.0), where the speed limit rises to 25 mph, a curve northeast and then just west of north, past a 2,078 ft. siding at Wilbur (MP 581.4), a detector at MP 583.0, a 4,615 ft. siding at Sutherlin, where a line to Hawthorne once headed east, and a curve northeast and then north-northwest at Oakland, where there is a 2,380 ft. siding.
There is a curve back to the north-northeast, a detector at MP 592.0, a gentle curve north past Isadora, a turn northeast at the 2,935 ft. siding at Rice Hill (MP 597.5), followed by a turn east and then north again, a detector at MP 602.3, the 3,405 ft. siding at Yoncalla (MP 603.7), a curve northwest, north again, northwest again, Drain (MP 609.0), a turn northeast, a spur at Krewson (MP 610.2), a turn east-northeast, a 3,092 ft. siding at Safley (MP 613.2), a turn northeast at Anlauf, past Curtin and Comstock, and a turn east to a summit and the 4,180 ft. siding at Divide (MP 621.9).
The line turns northeast again, past a detector at MP 623.2, Kimwood (MP 624.3) and Latham (MP 625.0), where the Coast Fork of the Willamette River comes alongside to the east. There is a 2,820 ft. siding at Cottage Grove (MP 626.5), where a logging railroad once left to the northwest and a line once headed east to Walden and beyond, and the line curves north, following the river, past a spur at Saginaw (MP 629.1) and a 3,249 ft. siding at Walker (MP 530.6), where the river edges away to the east. The line continues past the 3,137 ft. siding at Creswell, and Goshen (MP 640.9), where it turns northwest past a detector at MP 641.6), adjacent to to the south apex of the wye at Springfield Junction, and continues northwest to the junction itself.
At Springfield Junction (MP 644.4, el. 446 ft.), the ex-SP, now Central Oregon & Pacific, Siskiyou Line from Black Butte via Medford and Roseburg trails into the ex-SP (now UP) Natron Cutoff line, coming over the Cascades from the east, at a wye. CORP has trackage rights over the ex-SP main line the rest of the way into Eugene Yard.