"I believe that when you took these photographs, it was even darker than shown in the pic on the right below."
It was pretty dark but it got better as the morning proceeded. Never really good photography light and of course they all went through Lightroom.
At one time Kansas City Union Station was a bustling place. Trains arriving and departing at all hours of the day. Hundreds if not thousands of people there at all hours, too. The Kansas City massacre happened there in 1933. It was a shootout and killing of four policemen and a criminal named Frank Nash who was a federal prisoner. At that time period KC was big time in the mob and gang era.
When we got there in the early morning hours, we were the only people there. It is quite a spooky place at that time in the dark. The lights are all on timers so things come on all by themselves and not all at once either! They have a huge model train layout also on timers. We were in Santa's playground. Yeah, fun, fun!
The others were busy setting up recording equipment so I had to wander around and shoot some stuff. He brought some big lights that certainly helped.
"I am more interested in knowing whether the zoom's image quality (*) matches the quality offered by the two prime lenses when comparing at the same focal length."
To satisfy my curiosity in the above mentioned quote, i searched some more at https://www.the-digital-picture.com and found the "Image Quality" tool, which gives answers to my questions.The prime lenses in the comparison were 35mm f2 IS USM, 50mm f1.8 STM, and 85mm f1.8 USM (no IS).
You can go to the mentioned web site to check more if you're interested. I am posting what I found here so just in case anyone has similar questions than I had would know where to go to check.
As you can see in the screenshots below for the 85mm focal at f4, the prime lens easily beats the L-series zoom in sharpness and chromatic aberration (CA). I checked at other f5.6, f8 ... too. I did the similar checks at 35mm and 50mm. I found that the 35mm (with IS) also beats the zoom at same apertures. Only at 50mm f8 that the zoom seems to be equal or sharper than the prime lens at the center.
So absolutely speaking, the prime lenses still hold their advantages in term of sharpness and CA. For the prime lenses fans like me, don't get too excited until you read all the way to the end of this message.
Left pics are of the 24-105mm zoom and the right ones are of the 85mm:
Personally, I still like the prime lenses for the above reasons and for offering a bright viewfinders thanks to their wide apertures. And if you had read the low-light condition in which ebiggs1 had to take his pics at the Kansas City Union Station then you understand why he had a 85mm f1.2 !! I personally don't like flash and enjoy taking pics in low-light condition as well, in that sense, I like the prime lenses.
For folks who don't want to buy the 24-105mm L-series zoom, it's still time to stop reading.
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So, although the prime lens 85mm f1.8 USM seems to win based on the above demonstration, we need to be reminded that the above tests were most probably done in a lab with no cold wind and the camera was sitting comfortably on a tripod. In real life, I would be holding the camera with my hands and if there's a lot of wind and cold, my hands will shake. The bad news is the 85mm f1.8 USM features no Image Stabilizer (IS).
On the other hand, the 24-105mm f4 features IS all the way from 24 to 105 mm !!! That's a big advantage to compensate for the smaller f4 aperture.
So at this point, while I am still pondering, but I am leaning more and more towards the zoom despite my preference for prime lenses.
"I don't go by test charts done in laboratories. I never have liked them. I prefer to do the test myself in real use conditions. Usually turns out to be far different."
If I could justify to buy all the lenses, then yes, i would do my own tests too. But I can't justify buying them all.
For now, those charts satisfy my curiosity. 
"The problem here is many. First the tests are done in a laboratory, not in the real world which can be quite different. Some testers have an agenda. They may have a passion for Nikon for example."
Yes, it's hard to find good and solid information nowadays, and that's why I try to be very careful with anything I read on the internet. At this particular website, the author recommends a few times the Canon line of lenses for many reasons and one example is shown in the screenshot below. Moreover, the comparison that I was looking at was between two Canon lenses. So in this case, I think there is no Nikon-bias interference here.
"Although, my case is similar, but testers are testing a single copy of a given lens. That isn't sufficient to justify or condemn the entire model line."
Unless we are part of the Canon Quality Control department, I don't think anyone would be able to buy let's say 20 copies of a given lens to do their tests.
"Of course, it didn't hurt that I worked for a large company that had every lens one could want at our disposal."
Yes, so you understand that for the large majority of us amateurs out there, we need to rely on others who have done the tests. We need to see if a test was done without bias or agenda. With a little perseverance, we could find good info.
"As a side note the 85mm f1.2 is the exact same lens as the almost mythical Canon ef 50mm f1. That's correct f1. The FL is the only difference. Most of us will never even see one of them let alone own one."
Those two are dream lenses for low-light conditions, but the DOF must be so shallow that I would have a hard time to use them well.
