I have good and bad news.
First, the bad news: one of my good flaws is no longer. Take a look at the 2 short videos below of my Williams NW2 pulling a flat car unit train in each direction through the reverse loop in the vicinity of the voltage drop:
I'm teasing you, that's not bad news, that's good news! The train now runs smooth and steady through the reverse loop in each direction and there is no, or hardly any, voltage drop. I'm ecstatic!
How did I eliminate the voltage drop?
Scottie said to check to see if there are any loose connections between the track, and Don Winslow said that too and recommended special pliers with a notch for tightening tubular track connections. That tool looked familiar, I searched my tool box and, lo and behold, look what I found:
So, I used a very pliers with the same notch for tightened the track connections, and it worked.
Also, regarding measuring the voltage, I was able to do it, because I could use something that was staring me in the face while running the trains: my MTH Z4000 transformer. As most of you know, it tells you the amount of volts when running a train.
I set the transformer at 10 volts and ran the train through the entire outer loop of the layout. The Z4000 told me the voltage would vary between 9.4 and 10 volts, and when running over the track at issue, the voltage ranged between 9.6 to 10 volts. IMO, that is a very narrow range and the operation of the train with hardly any reduction in speed is a great improvement.
Thank you so much, Scotie and Don Winslow, for your good advice. Also, I thank RickO, GunrunnerJohn, D&H 65, Melgar and the rest of you for your good advice. I learned from all of you.
I think one can think of model railroad flaws in different ways. For instance, I started out stating that the flaw causing the reduction in speed of the train in that section of my layout is a good thing. There is some truth to that, because the train is running through town as the crossing gates go down. However, it is obviously also a very good thing to fix the flaw, eliminate or greatly reduce the voltage drop, and improve the operation of the train.
I guess that is just another thing that makes our hobby so interesting. Arnold