You can put multiple carts on a track, but they will collide if going opposite ways when pushed/powered. I don't know if that also happens when guided. The typical solution is a round trip track.
I'm the one who mentioned to starver that things changed in 0.40.19 (I think), so it's not an independent data point, but yes, jobs are posted to the job board and dorfs take them based on some counter intuitive and not fully understood rules.
- For the same job type, priority seems to take priority (except for some bug cases), while geometric distance (possibly with a weight factor for the Z axis) is used, although I'm not clear of over which distances the algorithm uses when there's a dorf position, an item pickup position, and a drop off position.
- For tasks where the target effectively is an area (such as the possible positions around a tree trunk to be cut) the geometrically closest one is taken, including when that single tile isn't part of the burrow or when the dorf actually has to pass through the other eligible tiles to reach the position (to build a wall from the wrong side).
- For tasks of different kinds the logic is completely muddled. Harvesting beats anything (although hauling to the hospital might have a higher priority), capable of cancelling a dorf's 3 day trek to perform a priority 1 digging activity.
- Hauling away old bones beats hauling of freshly slain corpses (actually, they're probably at exactly the same priority, leading to dorfs filing past the carcass as they head toward the old bones that were geometrically closer when they took the task) as well as fresh meat. The meat reaches priority parity when the meat has rotten...
- Construction (walls, floors, etc.) has a lower priority than refuse hauling, as does bridge building and tree felling (regardless of priority).
- Some hauling appears to have higher priority than workshop jobs.
- Trade depot hauling beats even refuse hauling in priority.
- There doesn't seem to be any priority given to corpse bits of sapients (destined for the corpse stockpile) over refuse: they seem to have the same (high) priority, so there's no attempt to reduce the corpse view shock impact as early as possible.