To be fair, UP did have their eyes on SP for a LOOONG time.
They were waiting for a time to pounce, and they did so right as they were coming out of the failed SPSF merger.
Let me tell you something you might already know.
"But did they have a lot of influence and power? SP and PRR were the only ones that would be considered as elite. They were always fighting for the top spot as far as the number 1 RR in the US."
No, they didn't. In the 1960s, every railroad was struggling to keep onto passenger service. It's not a form of rail travel that makes money. Freight is what makes money. So, to save the railroads from losing everything, the government bailed them out in 1970 with Amtrak. SP in there prime in 1970s? Hell no. Try 1940s-1960s. And even in the late 1980s to merger. The 1970s, I'll admit, were the most dramatic times in SP's life as a railroad. You had so many different locomotives, the iconic bloody nose was in it's prime, etc, etc. However, look closer. SP was struggling and wasn't trying too hard. Back in the 1970s, you couldn't tell if the crossing signal was on, or working, because they NEVER cleaned the lenses. So they looked like they were always off.
SP had dirty locomotives. They weren't running the best. They looked cool being dirty, but you know what, wouldn't want to run on SP in the 1970s.
To show you what I mean:
1976:
Look how dirty everything is, how warped and bumpy the track is, and how many weeds there are.
1977... wouldn't call this the prime time of these locomotives:
Now, don't get defensive with me. I just wanted to show you another side of SP you don't appear to have seen. Yes, I love an ALCO in rust, but in reality, they got scrapped in a few years since that photo. SP was upgrading. Just like Santa Fe, they were getting GEs. No doubt if they were still around they'd have replaced a lot of locomotives with SD70ACes, and GEVOs. It was for the best for the railroad to die young.
Also, about UP, bad railroads haven't lasts since 1846.

If it's worth anything, when they merged, the paperwork was very messed up. In the end, technically, UP merged into SP, but eh. UP is what's on the locomotives.
Now, about PRR, you have left out a lot of railroads... including the one you hate. UP. UP was VERY power hungry. Even more so than PRR. Turbines, DD40s, Big Boys, Challengers, the list goes on! Then there was the C&O, who invented their own Turbine, along with N&W, and a few others! Sure PRR was a powerhouse, but they ruined with when they merged with NYC into PC... which was a huge failure. Why? At one time, PC put 5 locomotives on a 20 car train in hopes that at least one was running by time it finished it 20 mile journey... only one WAS running when it finished. True, Conrail did straighten things out.
BTW, one of the local Rail America Class IIs (TPW) leases an old SP SD45T-2.
Cheers,
Joshua