My first sentence had read "their high priority stack trains." But then Mike Worcester posted the question: "Question for all you smart train people on here — what’s the distinction between a “stack” train and an “intermodal” train? Are those terms interchangeable? Or is there a difference that can easily be pointed out? Thanks!"
After reading Trevor's answer, I changed "stack" to "intermodal" in my first sentence.
Trevor Young: Technically, on BNSF, true “Stack” trains are designated by an S symbol. These are usually all international containers, mostly 40’ and 20’ double stacks headed for the ports. Intermodal does refer to all of it, but Z’s and Q’s are the higher priority intermodal trains, not designated the same as S trains, hauling domestic containers, trailers, and UPS trailers and containers on the Z’s. Hope this helps!
Mike Jackson: There is a difference in between S,Q&Z trains. S ( stack) trains are of course intermodal just like the Q and Z. S trains are just not as important. Q trains are guaranteed service trains. Z trains are UPS/LTL trains. The Z’s are supposed to be the highest priority on the rails but some subs don’t run them right. The Z also has a priority level 1-9 with 9 being the highest. The Z will also have pigs on it.
[Some comments point out that just because S is not as important as Q and Z trains doesn't mean they are not important. Most other trains will be put in a siding for an S train. I wonder where Amtrak trains are in the priority list.]