As I recall, UP had taken up the tracks to Yakima sometime before the WCRC era. UP had some sort of switching/haulage contract with BN to serve their customers on a few portions of track left in the lower valley at Sunnyside and Grandview where there was a connection to BNSF's Granger sub., and in the Yakima area. When BNSF later devoured the WCRC, they got the agreement back, too. Now that CWRR does the switching in Yakima, they are also doing the UP side. BNSF does the Yakima Valley turn, that carries the UP loads to the Pasco yard for eventual turnover to UP.

The old UP line ran from Zillah east of I82, parallel to the old NP Sunnyside branch; it crossed both the river and I82 at Sunnyside Dam (Bridge still in place and was used jointly!) They crossed the BNSF main rr west of Parker, and then paralleled into Yakima. YVT was a UP subsidary.
Interestingly, they actually surveyed the line on to cross the Cascades at Cowlitz Pass...

As for UP style rail on the Moxee branch: who knows where the Temples got it. But when the branch was relayed, just before the sale to BNSF, UP track was long gone from the valley.