Former US Route 99 from the Klamath River towards the Oregon State Line originally passed through the community of Hornbrook via what is Hornbrook Highway. US Route 99 entered Oregon via what is now Jefferson Road where it climbed towards Siskiyou Pass. US Route 99 was aligned south from the Oregon State Line through a Rail Subway (in the blog cover photo) constructed during 1916. Much of the corridor of US Route 99 north from the Klamath River to the Oregon State Line was replaced in 1949 via a new route through Henley. The 1949 alignment of US Route 99 was largely rebuilt and upgraded to Interstate 5 which completed north of the Klamath River to the Oregon State Line during in 1974.
https://www.gribblenation.org/2021/09/former-us-route-99-from-klamath-river.html