Shun the nonbeliever!
I saw this coming. I still hoped for something that uses the post room better. Yours still has a lot of blank space.
I dont know about carts being equal to none cart labor.
With whats being told us and how it's being presented, it seems like one dwarf is going to be able to do the work of several dwarfs.
If that's true, that already saving us quite a few Dwarf Hours in hauling.
So even if Loading into a Cart and Unloading into a Cart takes roughly the same amount of Time, it's taking less dwarfs to do the same work. So, it's more efficient even if it's not faster.
I dont think that'll need to make dwarf slower or really need to change how many objects a dwarf can carry.(Even though how many objects dwarf can carry is being changed.)
But this is more a cause of them having multiple haul jobs merged than carts, so the dwarfpower hauling might be a better option than ever. In other words, this applies to both dwarf haulers and carts, so this does not give an advantage to the carts.
Let me briefly touch on one further implication. The rocks are currently hardest to haul, because they cannot stack in either the inventories, or the stockpiles. So a cart might have an advantage of carrying multiples whereas dwarf cannot, even with the bin upgrade. However stone hauling is virtually always a dead end job, where you clean up your mined room and the cart tracks become abandoned.
Now, you COULD use carts for other stuff, like hauling most everything else, but most of those stuff stacks into bins, barrels, bags, for which the dwarves already got a virtual tenfold upgrade. So where the carts would be most useful, a constant repeated use, you're not gonna have much need for them because dwarves already somehow managed those on foot, and with the upgrade, one dwarf can handle even more.
I don't think things are that complicated by default. The dwarves just know this is the cart they're supposed to put the rocks (or the glass mugs, or the fresh-caught fish, or whatever) in. When it gets where it's going, then they re-path from there. Changes in the track will doubtless confuse dwarves, but I'm sure some people will be able to figure out more complex systems that are more efficient.
Actually even less complexity is required here. Make junctions where dwarves can take any turn, but will take the one that they need the most; just like in the traffic designations. I would say that it is safe to assume that dorfs will switch their junctions as they see fit, even invisibly so, just like we assume that the caves are lit with torches even though we cannot see them in game. Same as I would not want to bother with setting torches on walls too much, I would not want to bother opening a track for that dorf who is too mentally challenged to know to switch his own junction when he comes to it. Think of the aforementioned Transport Tycoon! You can pile up a square with tracks that lead in all eight directions, and the train will take one of those as per the need.
However, with each track having those certain 4 directions... the way that I interpret it, each rail tile will have 4 internal directions which can be set on and off. It would be neat if these directions could be individually forbidden or made oneway... For instance, a direction from the west to east could be set to allowed, to forbidden, to oneway from east, and oneway from west. I imagine this being super simple, and would only need to be done at occasional keypoints.
Keep the thoughts coming! ♥