I was asked to study use of the Siskiyou Line in 1974 when it was still owned by the SP. It would have taken five hours longer than the Cascade Line and it's probably slower now.Seriously doubt they'd detour over CORP's Siskiyou Line. Much of it is 10 mph. It would take a very long time.
SP ran local passenger trains over the line until the late 1950s. There were two, one ran from Black Butte junction to Medford, the other Eugene-Medford. One was named Rogue River, forget the name of the other one.
Sometime in the 1990s I think ODOT ran a demonstration train from Eugene as far as Roseburg, maybe even to Medford. It didn't go over Siskiyou Summit, though.
The last SP passenger train over the Siskiyou Summit, running Dunsmuir<>Grants Pass, had no name. It was replaced with a Pacific Greyhound (SP) bus connection in 1952. The Rogue River, running Portland<>Ashland ran till February 1955.
In earlier years, Amtrak used the Inside Gateway line for detours but I don't know of them using the Sacramento<>Klamath Falls segment since the UP got control of the WP.