The track from Landco (which is immediately east of Jastro siding) on the BNSF to Oil Junction
on the UP was never owned by the UP. It was built by the Santa Fe in the 1920s as part of its Minkler Southern Railroad project and has been operated by the San Joaquin Valley Railroad since December 1992 to access the Oil City branch and the joint trackage (with the UP) between Oil Junction and Richgrove.

The line has never been used for Amtrak detours; to my knowledge, no Amtrak train has ever detoured between Bakersfield and Fresno on the SP/UP (unlike between Fresno and Stockton). Nowadays, if there is an incident that shuts down Amtrak service on the BNSF in the valley, passengers would be bused.

There have been two times that fires on the SP's trestle over the Kern River resulted in SP freight trains detouring via the Santa Fe through downtown Bakersfield and subsequently on the Landco-Oil Junction track, the last time occuring in January 1992.

Rosedale Highway is one access point. Also, Standard Street just west of Highway 99 gets sort of close to the track (my Bakersfield map shows a couple of side streets off of Standard that get even closer but I haven't poked around in that area for quite awhile so I can't vouch for accessability).

Yes, there is a UP yard and a BNSF yard in Bakersfield (railfans should not use the term "railyard," which is largely the invention of ignorant newspaper reporters. Since "rail yard" is a short form of "railroad yard," it should be written as two words in my opinion).

Finally, Kern Junction is no longer an interchange point as SPGuy indicated. It remains a junction point, of course.