No train (or ship) of mine is scheduled by LTN to load more than 800k units of fluid, despite having both capacity and stations with request thresholds in millions
Is this intended behaviour?

Hi,

Thanks for the report. There is nothing in the code (at least that I can think of) that would cap this. What I would need is

• Version of Factorio and LTN (preferably Factorio 2.1, now that it is stable)
• A save for downloading that shows the problem.

Sure.

LTN is 3.4.0
I also have LTN Combinator at 2.5.1, if that's of importance
Factorio on 2.1.23

Where should I upload the save? It's 121 MB.
Do I need to provide my start-up settings or mod list, or is it baked into save data?

just drop the save onto a google drive or some file service. As long as there are not any private / uncommon mods in there, I can just load it and the game will download your mods (the list is in the save)

https://www.mediafire.com/file/uyjvbdrxq0irbbd/Rokkit_Science_49_NAUVIS_RERELOADED.zip/file

All mods here should've been only from portal.
If all loaded correctly, both legendary tanker and legendary four fluid car train should hold 2.2 million units of liquid

This is fixed in LTN 3.4.1 (just released). The code still used the "old" 2.0 API to calculate per-wagon fluid capacity and that API changed in 2.1. Now it correctly reports the capacity of the wagon (s) and the dispatcher matches the larger capacity (2.2m fluid).