Edited to add: 1.) Full disclosure, I've read some of my replies in the comment threads to this post. I was in a sour mood, and I coped by being a jerkface to commenters who legit didn't deserve that.

2.) That said, it is also becoming obvious to me that I'm not accurately describing the set of circumstances that I'm frustrated by. If someone might be willing to hit me up for a Discord call, so that either as multiplayer or by screensharing, I can literally "show you" my problem. NOTE: EVEN THOUGH someone doing that with me might not have a solution to my actual problem, through the act of walking someone through it I MIGHT STILL BENEFIT in coming up with a more succinct way of communicating my issue -- because it's obvious to me that the reason all of these very kind and well-intended replies (including a reply from a significant Factorio-YouTuber) are just not helpful because they address the words that I typed that obviously don't encompass a good understanding of my problem.

~~~

No matter what I'm trying to accomplish with fluids, the single point of failure for one hundred point zero percent of my attempts has been the unloading station, one tanker wagon will ALWAYS hold the otherwise empty train at the station for several minutes. I've tried using combinators to turn the train station on and off (which is a pain in the ass with LTN mod) I've redesigned my refineries, I've spent easily 10% of my 2200 Factorio hours trying to get fluid-by-train to actually work.

There is no size of fluid buffer that will do anything but mask and delay the inevitable. The first train to deliver to a bran new station always unloads fine, because the system doesn't get pressurized until something is delivered. Sometimes the problem affects the second train, sometimes not until the tenth train. But there is ALWAYS a train that gets delayed because one of the four tankers is still not empty. Turning the station off to allow the buffers to drain isn't a viable solution.

Please help.