This would have to confirmed by the experts but the UP CHI-OMA ex-C&NW Main has ATC (Automatic Train Control), a 1920's form of PTC that has both a train stop and a cab signal system. I am not sure how PTC has changed or replaced it on this line. This system has always required special equipment on the Engines being operated from, lead but not trailing units. to the reads in Chicago then near Omaha, perhaps the UP Power is added at the Western Ave., when it jumps from the CP/ Metra to the UP and then the ATC ends/ begins at Fremont Jtc. at those points the UP power can be dropped? I agree in my observation on any other Amtrak detour we would still get the GPS reads, so the UP Unit in lead is my theory.